^>, 


v.^. 


g; 


i^ 


I 


r-i 


1' 


.£     I 


.^'"> 


IMAGE  EVALUATION 
TEST  TARGET  (MT-3) 


1.0 


I.I 


US  ^^ 

itt  iiii   12.2 


Fholographic 

Sdmoes 

CarpojcBton 


1-25  1 1.4  pi.6 


CIHM/ICMH 

Microfiche 

Series. 


CIHM/ICMH 
Collection  de 
m 


Canadian  Inttituta  for  Historical  IMicroraproductions  /  institut  Canadian  da  microraproductions  liiatorlquas 


Technical  and  Bibliographic  Notas/Notas  tachniquas  at  bibliouraphiquaa 


Tha  Institute  has  attamptad  to  obtain  tha  bast 
original  copy  availabia  for  filming.  Faaturas  of  this 
copy  which  may  ba  bibllographlcally  unique, 
which  may  alter  any  of  tha  images  in  the 
reproduction,  or  which  may  significantly  change 
the  usual  method  of  filming,  are  checked  below. 


P 


D 
D 


D 


Coloured  covers/ 
Couverture  de  couleur 


I      I   Covers  damaged/ 


Couverture  endommagAe 

Covers  restored  and/or  laminated/ 
Couverture  restaurte  at/ou  pallicuMe 

Cover  title  missing/ 

Le  titre  de  couverture  manque 

Coloured  maps/ 

Cartes  gAographiques  en  couleur 

Coloured  ink  (i.e.  other  than  blue  or  black)/ 
Encre  de  couleur  (i.e.  autre  que  bleue  ou  noire) 


I     I   Coloured  plates  and/or  illustrations/ 


Planches  et/ou  illustrations  an  couleur 

Bound  with  other  material/ 
Rail*  avec  d'autres  documents 

Tight  binding  may  causa  shadows  or  distortion 
along  interior  margin/ 

La  raliure  serrAa  paut  causer  de  I'ombre  ou  de  la 
distortion  la  long  de  la  marge  IntMeure 

Blank  leaves  added  during  restoration  may 
appear  within  the  text.  Whenever  possible,  these 
have  been  omitted  from  filming/ 
II  se  peut  que  certalnes  pages  blanches  ajout^as 
lors  d'une  restauration  apparaissent  dans  le  texte, 
male,  lorsque  cela  Atait  possible,  ces  pages  n'ont 
pas  4t4  filmAas. 


L'Institut  a  microfilm*  le  meilieur  exemplaire 
qu'il  lui  a  At*  possible  de  se  procurer.  Les  details 
da  cat  exemplaire  qui  sont  peut-Atre  uniques  du 
point  de  vue  bibllographique,  qui  peuvent  modifier 
une  image  reproduite,  ou  qui  peuvent  exiger  une 
modification  dans  la  mAthode  normala  de  filmaga 
sont  Indiqute  ci-dessous. 


D 

J? 
D 

D 

D 

D 

D 

D 

D 

D 


Coloured  pages/ 
Pages  de  couleur 

Pages  damaged/ 
Pages  endommagtes 

Pages  restored  and/or  laminated/ 
Pages  reataurtes  et/ou  pellicultes 

Pages  discoloured,  stained  or  foxed/ 
Pages  dAcolortes,  tachetAes  ou  piquAes 

Pages  detached/ 
Pages  d6tachAes 

Showthrough/ 
Transparence 

Quality  of  print  varies/ 
Quality  inAgale  de  I'lmpression 

includes  supplementary  material/ 
Comprand  du  matArldl  suppMmentaire 

Only  edition  available/ 
Seule  Mition  diaponible 

Pages  wholly  or  partially  obscured  by  errata 
slips,  tissues,  etc.,  have  been  refilmed  to 
ensure  the  best  possible  image/ 
Lea  pages  totaiement  ou  partiellement 
obscurcies  par  un  feuillet  d'errata,  une  pelure, 
etc.,  ont  4t*  filmtes  A  nouveau  de  fa^on  A 
obtenir  la  meilleure  Image  possible. 


Additional  comments:/ 
Commentairea  supplemental 

res: 

This  item  is  filmed  at  the  reduction  ratio  checked  below/                                                                                                  ■ 
Ce  document  est  filmA  au  taux  de  rMuction  indiquA  ci-deaaous.                                                                                       ^ 

10X                           14X                           18X                           22X                           »X                          aOX 

1 

C 

X 

1 

12X 


1«X 


aox 


MX 


2BX 


ax 


tails 

du 
odifiar 

una 
maga 


Tha  copy  filmad  hara  has  baan  raproducad  thanks 
to  tha  ganarosity  of: 

Library  of  Congrass 
Photodupiication  Sarvica 

Tha  imagas  appaaring  hara  ara  tha  bast  quality 
posslbia  considaring  tha  condition  and  lagiblllty 
of  tha  original  copy  and  in  kaaping  with  tha 
filming  contract  spacif icatlons. 


L'axamplaira  film*  fut  raproduit  grAca  A  la 
gAnArositA  da: 

Library  of  Congrass 
Photodupiication  Service 

Laa  imagas  suivantas  ont  At4  raproduitas  avac  la 
plus  grand  soin,  compta  tanu  da  la  condition  at 
da  la  nattatA  da  I'axampiaira  filmA,  at  an 
conformity  avac  las  conditions  du  contrat  da 
filmaga. 


Original  copies  in  printed  paper  covers  ara  filmad 
beginning  with  the  front  cover  and  ending  on 
the  last  page  with  a  printed  or  illustrated  impres- 
sion, or  the  back  cover  when  appropriate.  All 
other  original  copies  are  filmed  beginning  on  the 
first  page  wKh  a  printed  or  illuetratad  impres- 
sion, and  ending  on  the  last  page  with  a  printed 
or  illustrated  impreasion. 


IS 


Lee  exemplaires  originaux  dont  la  couverture  en 
papier  est  ImprimAe  sont  filmte  en  commen9ant 
par  la  premier  plat  at  en  termlnant  soit  par  la 
darnlAre  page  qui  comporte  une  empreinte 
d'Impresslon  ou  d'illustration,  soit  par  la  second 
plat,  selon  le  cas.  Tous  las  autrea  exemplaires 
originaux  sont  filmte  en  commen^ant  par  la 
pramlAre  page  qui  comporte  une  empreinte 
d'impression  ou  d'illustration  at  an  termlnant  par 
la  darnlAre  page  qui  comporte  une  telle 
empreinte. 


The  laat  recorded  frame  on  each  microfiche 
shall  contain  the  symbol  -^  (meaning  "CON- 
TINUED"), or  the  symbol  ▼  (meaning  "END"), 
whichever  applies. 


Un  des  symboles  suivants  apparattra  sur  la 
darniire  image  de  cheque  microfiche,  selon  le 
cae:  le  symbols  — ►  signifie  "A  SUIVRE",  le 
symbole  V  signifie  "FIN". 


Maps,  plataa,  charts,  etc.,  may  be  filmed  at 
different  reduction  ratioa.  Thoae  too  large  to  be 
entirely  included  in  one  expoaure  ara  filmed 
beginning  in  the  upper  left  hand  corner,  left  to 
right  and  top  to  bottom,  as  many  framaa  as 
required.  This  following  diagrama  illustrate  the 
method: 


Lea  cartas,  planchae,  tableaux,  etc.,  peuvent  Atre 
filmte  A  des  teux  de  reduction  difftlrents. 
Lorsque  le  document  est  trop  grand  pour  Atre 
reproduit  en  un  saul  cliche,  11  aet  f llmA  A  partir 
da  Tangle  supArleur  gauche,  de  gauche  A  droite, 
et  de  haut  en  bas,  en  prenant  la  nombre 
d'lmagaa  nteassaira.  Lea  diagrammes  aulvants 
illustrant  le  mAthode. 


errata 
I  to 

t 

I  pelure, 

on  A 


■mmmbbI 


1  2  3 


ax 


1 

2 

3 

4 

5 

6 

:¥'■.- 


■J.. 


6^  i'/. 


.MS 


REPORT 


TO  TUK 


^•!*»'^1'^'V 


SECRETARY  OF  THf  1 AVY 


ui( 


RECENT  IMPROVEMENTS  IN  ASTRjIpim  INSTRMNTS: 


■-:.? 


','1  ■1' 
>■'■.( 


:)| 


■I"' 


By   SIMON    NF.WOOMB, 

FBOVBBROB,  W.  «•  WAVV.      ,  --    ' 


HHMTOfi^MMM 


'        i884.        -      ^  ' 


,    ) 


L,i£^ 


f    -M 


■-t..V  ■•■.v.*-»^*Me^?f?^*^rrv-r^rV'SP->-v  }-<■'' 


■  \i*-,  i''.v,,vt>'r  ■  ."r^."--_:i(A,-..'^.!-r,  .-av-.':^.- V  - 


■I 


.  u. 


1      ft      _jf.v  "i 


%f^ 


'W 


i'l^S 


I 


t 


REPORT 


TO  THa 


SECRETARY  OF  THE  NAVY 


oir 


RECENT  DIPROYBMENTS  IN  ASTRONOMICAL  INSTRUMENTS. 


Bv  SIMON  NEWOOMB. 
noraasoB.  v.  &  vxrr. 


"WASfllNQTOK: 

OOTBBIfMSRS  PUIKTIKO  0»»101. 

1884. 


*^  '.aMK'iwaai&ffin  wjwaaMMw^wiwwi*^^ 


•»•■■■.    ■: 


I 


46th  Congress, 
1st  Session. 


SENATE. 


Ex.  Doo. 
No.  96. 


MESSAGE 

FROM   THE 


PRESIDENT  OF  THE  UNITED  STATES, 


TRANSMITTING 


i "    A  communication  from  the  Secretary  of  the  Navy,  including  a  report  on 
recent  improvements  in  astronomical  instruments. 


Fbbruary  12,  1884.— Read  and  referred  to  the  Committfle  on  Naval  Affairs  and 

ordered  to  be  printed. 


To  the  Senate: 

I  transmit  a  communication,  under  date  of  the  8th  instant,  addressed 
to  me  by  the  Secretary  of  the  Navy,  covering  a  repoi;t  of  Prof.  Simon 
Newcomb,  United  States  Navy,  on  the  subject  of  recent  improvements 
in  astronomical  observatories,  instruments  and  methods  of  observation, 
as  noted  during  his  visit  to  the  principal  observatories  of  Europe,  in 
the  year  1883,  made  iu  pursuance  of  orders  of  the  Navy  Department. 

The  request  of  the  Secretary  is  commended  to  the  consideration  of 
Gongress. 


ExBCUTivB  Mansion, 

February  11, 1884. 


CHESTER  A.  ABTHUB. 


II 


JTavy  Department, 
y\/,i3hington,  February  8, 1884. 
Sib  :  I  have  the  honor  to  transmit  the  report  of  Prof.  Simon  Newcomb, 
United  States  Navy,  on  recent  improvements  in  astronomical  observa- 
tories, instruments  and  methods  of  observation,  as  noted  during  a  visit 
to  the  principal  observatories  of  Europe  in  the  spring  and  summer  of 
1883,  made  in  pursuance  of  orders  of  this  Department.    The  report  is 
considered  of  sufficient  importance  to  be  laid  before  Congress  and  printed . 
Very  respectfully,  your  obedi^t  servant, 

WM.  E.  CHANDLER, 

Secretary  of  the  Navy. 
The  Pbb8idbnt« 


■Immm 


iiiniiniiiiiiiiiiii 


IMPROVEMENTS   IN   ASTRONOMICAL   INSTRUMENTS. 


mA 


Nautical  Almanac  Office,  Navy  Department, 

'Wcaihxngton^  I),  C,  January  20, 1884. 

Sir:  In  pursuance  of  onlers  from  the  Department  I  visited  certain  of 
the  leading  observatories  on  the  continent  of  Europe  during  the  past 
year  for  the  purpose  of  collecting  information  respecting  the  most  recent 
improvements  in  astronomical  instruments  and  methods  of  observation. 
I  now  have  the  honor  to  submit  the  following  report  upon  the  knowl- 
edge thus  gained.  The  heads  of  this  report  are  not  arranged  with 
respect  to  the  different  establishments  visited,  but  with  respect  to  the 
different  kinds  of  instruments,  all  that  relates  to  each  instrument  being 
collected  together,  even  when  the  material  was  gathered  at  various 
places. 

The  establishments  visited  fh>m  which  valuable  information  was  gath- 
ered, were  the  observatories  of  Paris,  Neuchatel,  Geneva,  Vienna,  Ber- 
lin, Potsdam,  Leyden,  and  Strassburg,  and  the  workshop  of  the  Messrs. 
Bepsold  at  Haml>urg.  At  the  latter  place  I  enjoyed  the  opportunity  of 
meeting  Director  Struve,  of  the  Pulkowa  Observatory,  and  of  discuss- 
ing with  him  and  the  Bepsolds  the  plans  of  the  great  30-inch  refhiotor, 
the  objective  of  which  had  just  been  completed  by  the  Messrs.  Olark. 

It  is  both  a  duty  and  a  pleasure  to  acknowledge  the  very  cordial  re- 
ception I  met  from  the  directors  and  astronomers  of  the  various  obser- 
vatories, and  the  facilities  which  were  everywhere  afforded  me  for  the 
execution  of  the  mission  with  which  I  was  charged.  In  every  case  the 
fullest  liberty  was  accorded  me  to  make  as  critical  an  examination  of 
every  iwint  as  circumstances  permitted. 

In  this  report  it  is  not  practicable  to  present  that  exhaustive  discus- 
sion of  the  subject  of  recent  instruments  which  might  have  been  ex- 
pected, for  the  reason  that  within  the  limited  time  at  my  disposal  it  was 
not  possible  to  prepare  the  detailed  drawings  and  make  the  tests  which 
would  have  been  necessary  for  that  purpose.  I  shall  therefore  confine 
my  report  to  such  special  points  as  appear  most  important  to  persons 
who  may  intend  to  found  new  observatories,  or  to  design  or  purchase 
new  astronomical  instmments. 

THE  great  yiENNA  TBLBSOOPB. 

Among  the  InstmmentB  which  I  have  examined,  that  to  which  most 
interest  now  attatches  is  the  great  telescope  recently  completed  for  the 
Imperial  Observatory  at  Vienna,  by  Howard  Ombb,  esq.,  of  Dublin. 
It  is  the  largest  refracting  telescope  in  actual  use  at  the  present  time, 


RUMENTS. 
iSPABTMBNT, 

January  29, 1884. 
1 1  visited  certain  of 
3pe  dnrlog  the  past 
itinjc  the  most  recent 
hods  of  observation, 
art  upon  the  kuowl- 
I  not  arranged  with 
with  respect  to  the 
ich  instrument  being 
gathered  at  various 

aformation  was  gath- 
Qeneva,  Vienna,  Ber- 
rkshop  of  the  Messrs. 
ed  the  opportunity  of 
fctory,  and  of  disouss- 
reat  30-inch  reflractor, 
by  the  Messrs.  Olark. 
»  the  very  cordial  re- 
of  the  various  obser- 
B  afforded  me  for  the 
d.    In  every  case  the 
cal  an  examination  of 

at  exhaustive  discus- 
might  have  been  ex- 
at  my  disposal  it  was 

make  the  tests  which 
hall  therefore  confine 

important  to  persons 
to  design  or  purchase 

Bd,  that  to  which  most 
ntly  completed  for  the 
^mbb,  esq.,  of  Dublin. 
se  at  the  present  time, 


IMPROVEMENTS  IN  ASTRONOMICAL  INSTRUMENTS.       8 

being  of  one  inch  greater  aperture  than  that  of  the  Naval  ObHcrvutory 
at  Washington.  The  contract  waM  made  with  Mr.  Grubb  in  1875,  but, 
owing  to  diflBculties  in  procuring  glass  disks  of  the  necessary  size  and 
purity,  it  was  not  completed  until  1881.  Further  delay  occurred  in 
mounting,  so  that  it  was  barely  ready  for  active  work  at  the  time  of  my 
visit  in  April  last.  I  made  as  careftil  and  critical  an  examination  of  its 
working  as  was  possible  during  the  unfavorable  weather  which  pre- 
vailed at  that  time  at  Yien.ia.    My  examination  was  principally  in  the 

nature  of  a  comparison  of  its  working  with  that  of  the  Washington 
telescope. 

General  etyle  of  mounHng.  —  In  its  main  features  the  telescope  is 
mounted  on  the  same  fundamental  plan  as  that  at  Washington,  each 
being  on  the  (German  plan,  with  the  Gtorman  system  of  counterpoises 
and  with  a  steel  tube.  In  both,  the  rapid  motion  in  declination  is  by 
means  of  a  rope  attached  to  the  two  ends  of  the  tube;  that  in  right  as- 
cension by  a  system  of  wheel-work.  In  both,  the  clock-work  is  in  the 
pier  below  the  instrument.  The  leading  points  of  difference  are,  that 
the  mounting  of  the  Vienna  telescope  is  much  larger,  stronger,  and 
heavier  in  all  its  parts,  that  the  appllMices  for  using  it  are  more  elab> 
orate  and  numerous,  and  that  an  elaborate  system  of  friction  rollers  in 
declination  is  provided,  while  the  Washiflgton  telescope  has  none.  A 
more  convenient  system  of  illuminating  the  field  and  the  divisions  on 
the  several  circles  has  also  been  introduced.  As  a  piece  of  mechanical 
engineering  it  reflects  great  credit  upon  its  designer  and  constructor. 

Eaee  of  motion. — In  moving  the  Vienna  telescope  one  is  at  first  struck 
with  the  tact  that  mere  weight  is  a  serious  drawback  in  the  management 
of  such  an  instrument,  but,  when  the  motion  is  once  commenced,  the 
movement  in  right  ascension  is  almost  as  easy  as  in  the  Washington 
telescope.  It  is,  however,  very  different  in  declination.  For  reasons 
which  neither  Dr.  Weiss  nor  myself  was  able  to  perceive,  the  Motion 
rollers  seemed  to  be  of  little  benefit  in  easing  the  motion  in  declination, 
which  was  much  more  difficult  than  in  the  Washington  telescope,  and, 
in  &ot,  quite  a  tax  upon  the  strength  of  the  observer  at  the  eye-piece. 

The  quick  motion  for  setting  in  right'  ascension  is  made  below  the 
end  of  the  polar  axis  by  turning  a  steel  steering-wheel.  This  appliance 
is  in  every  way  inferior  to  the  system  at  Washington,  where  the  same 
motion  ia  effected  by  an  endless  rope  hung  over  a  grooved  wheel  which 
the  observer  palls  hand  over  hand.  By  this  axnuigement  the  observer 
at  the  Washington  teleooope  can  make  the  required  motion,  with  his 


IMPROVEMENTS   IN   ASTRONOMICAL    INSTRUMENTS. 


'- :.  'if  • 


iH'il 


■-*'! 


eyes  fixed  upon  the  tcIcHcope  or  upon  the  vernier,  as  he  maj'  desire,  and 
without  giving  any  thought  to  the  motion  of  the  hands.  But  the 
handles  of  the  steel  wheel  are  much  less  convenient  to  take  hold  of  than 
a  rope ;  and  if  the  motion  is  at  all  rapid  the  operator  cannot  turn  his  eyes 
to  the  moving  tele8coi»e  without  danger  of  his  knuckles  being  struck 
by  the  steel  handles  as  he  attempts  to  take  hold  of  them  without  look- 
ing. The  necessity  for  oare  in  this  respect  makes  the  motion  hesitating 
and  laborious,  at  least  to  one  unaccustomed  to  it. 

The  clock  motion. — On  the  system  of  the  Messrs.  Clark,  applied  in  the 
Washiugton  telescope,  the  screw  which  turns  the  sector  does  not  take 
hold  of  the  circumference  of  the  latter  directly,  but  gears  into  a  complete 
worm-wheel,  around  the  axis  of  which  is  wrapped  a  pair  of  brass  or  steel 
bauds  which  also  enwrap  the  arc  of  the  sector.  By  this  arrangement 
the  toothed  wheel  makes  a  nearly  complete  revolution  while  the  sector 
is  moving  through  its  arc,  and  the  effect  of  the  small  unavoidable  irreg- 
ularities in  the  working  of  the  screw  is  diminished  in  the  ratio  of  the 
arc  of  the  sector  to  the  circumference  of  the  wheel.  Whatever  advan- 
tages this  arrangement  may  have  in  small  Instruments,  I  think  that  in 
large  ones  they  are  more  than  counterbalanced  by  the  irregularities 
arising  from  the  elasticity  of  the  band,  combined  with  the  variations  of 
firiotion,  and  the  action  of  wind  and  other  forces  operating  to  vary  tbo* 
uniform  motion  of  the  telescope.  Owing  to  this  elasticity,  the  eftioct  of 
the  wind  or  of  any  slight  pressure  by  the  observer  on  the  eye-piece  is 
many  times  greater  in  the  Washington  than  in  the  Vienna  instrnmoat. 
But  it  did  not  appear  to  me  that  the  firmness  of  the  connection  in  the 
latter  instrument  between  the  support  of  the  turning  screw  and  the 
tube  of  the  telescope  was  as  great  as  was  expected  by  those  who  lay 
stress  on  large  and  stable  mountings.  I  found  that  by  a  simple  press- 
ure of  the  thumb-nail  upon  the  tube  of  the  Vienna  telescope  the  point- 
ing in  right  ascension  could  be  changed  by  several  seconds  so  as  to 
throw  an  object  entirely  away  from  the  wire. 

The  main  question  is,  however,  the  steadiness  of  motion  when  no 
pressure  whatever  is  applied  by  the  observer,  and  so  far  I  have  found 
no  large  telescope  which  is  entirely  satisfactory.  The  Vienna  telescope 
was  not  supplied  with  a  micrometer  at  the  time  of  my  examination, 
BO  that  I  could  not  test  its  motion  as  thoroughly  as  I  wished  to;  but 
by  bringing  the  planet  Uranus  in  the  edge  of  the  field  I  found  that 
when  the  clock  was  going  there  was  a  constant  irregular  movement  in 
right  ascension,  the  amount  of  which  I  estimated  as  between  one  and 


rRUMENTS.       • 

8  ho  mny  desire,  and 
u  hands.  But  the 
to  takehoUl  of  than 
cannot  tarn  his  eyes 
uvkles  heing  struck 
them  without  look- 
the  motion  hesitating 

Clark,  applied  in  the 
sector  does  not  take 
gears  into  a  complete 
A  pair  of  brass  or  steel 
By  this  arrangement 
ition  while  the  sector 
all  nnavoidable  irreg- 
ed  in  the  ratio  of  the 
b1.    Whatever  advan- 
ments,  I  think  that  in 
by  the  irregularities 
with  the  variations  of 
operating  to  vary  tho» 
elasticity,  the  effect  of 
ver  on  the  eye-piece  is 
;he  "Vienna  instrumont. 
if  the  connection  in  the 
turning  screw  and  the 
Bcted  by  those  who  lay 
that  by  a  simple  press- 
ana  telescope  the  point- 
everal  seconds  so  as  to 

less  of  motion  when  no 
and  so  far  I  have  found 
.  The  Vienna  telescope 
ime  of  my  examination, 
;hly  as  I  wished  to;  but 
if  the  field  I  found  that 
t  irregular  movement  in 
bted  as  between  one  and 


1MPR0VBMENT8    IN   ASTRONOMICAL    INSTRUMENTS.  5 

two  HcuundH  of  arc.  TIiIh  niovcnivnt  had  no  regiihir  p«>ri(Ml,  and  there- 
fore did  not  Heem  to  be  connected  witli  any  defect  in  the  flgnrc  or  mo- 
tion of  the  Hcrew.  Its  irre(;nhir  period,  if  I  may  use  the  term,  viiried 
from  the  smalleHt  nppreciublo  amount  to  two  or  tlir<>e  8euondH  of  time. 
Itti  nioHt  probable  cauHe  seemed  to  be  the  variable  friction  of  tlie  mo- 
tion in  riglit  ancenHion  and  CHpecially  of  the  friction  rollerH  by  which 
the  polar  axiH  is  Hiipporte<i  at  its  lower  end.  A  Mimilar  irregularity  is 
noticeable  in  the  Watihington  telescope,  but  when  the  conditions  nre 
favorable  it  is  less  than  that  noticed  at  Vienna.  On  the  other  hand, 
the  effect  of  wind  is  much  greater  in  the  cumc  of  the  Washington  tele- 
scope. 

Arrangement  of  Hector, — In  Mr.  Orubb*s  large  teleHcofte,  an  attempt  is 
made  to  give  greater  stability  to  the  8crew  by  having  the  ends  of  its 
axis  to  fit  into  firm  snpports  in  the  massive  base  of  the  telescope,  thus 
rendering  it  incapable  of  any  motion  except  that  of  turning.  The  screw 
cannot  therefore  be  unlocked  from  the  sector  as  in  the  instruments  by 
other  makers.  When  the  8e<;tor  reaches  the  end  of  its  motion,  it  has  to 
be  turned  back  by  giving  a  rapid  backward  motion  to  the  screw  itself, 
tor  which  special  apparatus  is  provided.  From  what  I  have  already 
said,  I  am  of  opinion  that  this  arrangement  offers  no  advantage  to  com- 
pensate for  the  trouble  which  it  causes  the  observer. 

Slow  motion. — The  slow  motion  in  right  ascension  in  the  Vienna  tele- 
scope is  endless,  instead  of  being  confined  between  narrow  limits  as  that 
at  Washington.  This  is  a  decided  improvement,  saving  the  observer 
much  loss  of  time  from  the  motion  running  out,  which  it  is  sure  to  do 
fh)m  time  to  time. 

Illumination. — The  apparatus  for  illuminating  the  field  of  the  mi- 
crometer was  not  in  perfect  order  at  the  time  of  my  visit,  so  that  I  can- 
not report  upon  it  in  this  connection.  It  is  in  its  general  character 
similar  to  the  system  adopted  by  the  Messrs.  Bepsold,  of  which  I  shall 
speak  hereafter.  The  illumfiiation  of  th6  divisions  of  the  setting  circles 
leaves  nothing  to  be  desire<l. 

Minor  points. — In  tfie  preceding  remarks,  I  have  indicated  what  may 
be  considered  fundamental  points  affecting  the  use  of  the  Vienna  tele- 
scope.  There  are,  however,  a  number  of  minor  points,  which  are  of 
almost  equal  importance,  so  far  as  the  practical  use  of  the  telescope  is 
concerned.  As  the  instrument  now  stands,  the  drawback  which  struck 
me  most  was  the  absence  of  any  rough  setting  either  in  right  ascension 
or  declination,  and  the  impossibility  of  seeing,  even  approximately,  the 


MMSmMV:  1«>'S34I8V'. : 


t!»«isa»»«tn(^' '.*"  ■ 


6 


IMPROVEMENTS   IN   ASTRONOMICAL   INSTRUMENTS. 


pointing  in  declination,  except  when  the  observer  was  at  the  eye-piece. 
This  want,  combined  with  the  great  force  necessary  to  move  the  tele- 
scope in  declination,  makes  its  pointing  a  diflBcult  and  troublesome 
operation.  The  observer  must  first  set  the  telescope  by  pure  guess-work. 
He  has  then  to  mount  to  the  eye-piece,  wherever  it  may  be,  look  into 
the  niicroscope,  and  note  the  reading  of  the  circle.  He  has  then  to  with- 
draw his  eye,  and  by  considerable  muscular  exertion  to  make  another 
guess,  which  he  again  tests  by  reading  the  circle.  Thus  the  pointing 
is  to  be  made  by  a  series  of  trials  which  are  so  troublesome  that  I  found 
the  observers  were  in  the  habit  of  mounting  to  the  top  of  the  cylinder 
of  the  dome  and  finding  the  pointing  in  declination  by  moving  the  tele- 
scope around  the  horizon. 

I  remark,  in  this  connection,  that  the  Washington  telescope  has  a 
coarse  setting  which  the  observer  can  read  from  any  point  below  the 
telescope  with  the  aid  of  an  opera-glass.  This  setting  is  sufficiently 
accurate  to  bring  any  object  whose  position  is  known  into  the  field  of 
view  of  the  finder,  and  near  its  center. 

Objective. — ^The  proper  fi^ring  of  an  objective  so  as  to  give  the  best 
possible  image,  is  justly  considered  th<)  most  difficult  task  in  the  con- 
struction of  a  large  telescope.  Especial  interest,  therefore,  attaches  to 
Mr.  Orubb's  success  with  the  objective.  The  atmospheric  conditions 
during  my  visit  were  unfavorable  to  the  finest  tests,  but  I  succeeded  in 
making  such  examination  as  the  cirouihstances  admitted  of  en  three 
evenings.  On  the  first  trial,  the  image  was  found  to  be  defective,  owing 
to  a  want  of  a4iastment  of  the  glass  itself.  This  was  corrected  next 
day  by  Director  Weiss.  On  the  second  trial  I  found  a  well-marked 
spherical  aberration,  which  seemed,  however,  to  be  very  regular  from 
center  to  circumference.  But  there  had  been  a  fall  of  temperature  and 
the  dome  had  been  opened  only  a  short  time;  circumstances  under  which 
the  Washington  telescope  always  exhibited  the  same  phenomena.  On 
the  third  evening,  the  dome  had  been  opened  long  enough  to  nearly 
equalize  the  internal  and  external  temperatures.  So  far  as  I  could  judge 
the  character  of  the  image  was  perfect,  there  being  no  appearance  of 
those  rings  of  different  focal  length,  which  are  so  often  found  in  large 
objectives.  As  I  hacl  not  used  a  large  telescope  for  some  eight  years,  I 
could  not  feel  that  my  judgment  was  of  the  most  critical  kind,  but  I  am 
persuaded  that  if  any  defects  exist,  they  are  so  minute  as  not  to  interfere 
in  any  important  degree  with  the  finest  performance  of  the  instrument. 

The  color  correction  is  less  than  in  the  Washington  telescope.  The 
result  is  that  the  blue  areole  around  brilliant  objects  is  much  less  striking. 


rRUMENTS. 

iras  at  the  eye-piece, 
py  to  move  the  tele- 
It  and  troublesome 
by  pure  guess-work, 
t  may  be,  look  into 
He  has  then  to  with- 
)n  to  make  another 

Thus  the  pointing 
blesome  that  I  found 

top  of  the  cylinder 
I  by  moving  the  tele- 

rton  telescope  has  a 
any  point  below  the 
etting  is  sufficiently 
»wn  into  the  field  of 

10  as  to  give  the  best 
cult  task  in  the  con- 
iherefore,  attaches  to 
nospheric  conditions 
»,  bat  I  succeeded  in 
idmitted  of  en  three 
l»  be  defective,  owing 
1  was  corrected  next 
found  a  well-marked 
be  very  regular  from 

11  of  temperature  and 
astances  under  which 
bme  phenomena.  On 
ng  enough  to  nearly 
3o  far  as  I  could  judge 
Dg  no  appearance  of 
)  often  found  in  large 
>r  some  eight  years,  I 
critical  kind,  but  I  am 
ate  as  not  to  interfere 
oe  of  the  instrument, 
igton  telescope.  The 
3  is  much  less  strikiug. 


IMPROVEMENTS    IN    ASTRONOMICAL    INSTRUMENTS.  7 

THE  GREAT  DOMES  AT  PARIS  AND  VIENNA. 

The  proper  perforoiauce  of  a  large  telescope  is  so  much  affected  by 
the  character  of  the  dome  in  which  it  is  placed,  that  the  latter  may  be 
regarded  as  of  equal  importance  with  the  mounting  of  the  telescope. 
All  my  experience,  however,  leads  me  to  the  conclusion  that  there  is 
no  decided  superiority  in  any  special  form  of  dome,  but  that  the  princi- 
pal difference  in  the  working  arises  from  the  quality  of  the  workman- 
ship. In  choosing  among  a  number  of  proposed  forms  we  can  only  say 
that  that  which  is  best  constructed  is  the  best. 

The  Vienna  dome  was  constructed  by  Mr.  Grubb.  It  is  built  of  iron, 
is  45  feet  in  external  diameter,  and  weighs  fifteen  tons.  Its  working 
leaves  nothing  to  be  desired,  except  that  its  great  weight  renders  its 
motion  somewhat  cumbrous.  By  moving  it  a  short  distance  it  appeared 
that  one  man  could  turn  it  in  eight  minutes.  Mr.  Grubb  says  that  when 
first  mounted  a  weight  of  seven  poands  on  the  rope  was  sufficient  to 
start  it.  I  did  not  test  this  by  actual  trial,  but  cannot  resist  the  con- 
clusion that  much  more  than  seven  pounds  is  now  required. 

The  drum  of  the  dome  is  of  thick  massive  brick- work.  I  cannot  but 
regard  this  sort  of  base  as  objectionable,  even  when  pierced  with  nu- 
merous openings,  as  in  the  present  case,  owing  to  the  difficulty  of  se- 
curing equable  temperatures  inside  and  outside. 

Heretofore  three  methods  of  supporting  a  turning  dome  have  been 
proposed: 

I.  Cn  wheels,  fixed  either  to  the  dome  or  to  the  base  on  which  the 
dome  rolls. 

II.  On  a  system  of  rollers  connected  by  a  live  ring. 

III.  On  cannon  balls. 

I  conceive  that  the  choice  should  lie  between  the  last  two.  The  can- 
non-ball system  works  the  best  of  all  so  far  as  ease  of  motion  is  con- 
cerned ;  its  drawback  is  the  difficulty  of  keeping  the  balls  at  anything 
like  equal  distances  apart.  The  system  of  rollers  in  a  live  ring  was  in- 
vented by  Mr.  Grubb,  and  is  employed  at  Washington  as  well  as  at 
Vienna.  It  has  the  advantage  of  always  working  well,  but  is  more 
troublesome  in  construction  and  requires  more  force  than  cannon  balls. 

To  these  three  systems  the  French  astronomers  propose,  in  their  new 
great  dome,  to  add  a  fourth,  by  floating  the  dome  in  an  annular  trough 
forming  the  top  of  the  drum.  The  base  of  the  dome  will  then  be  a  float- 
ing annular  caisson.    It  woald  be  hasardoas  to  predict  in  advance  how 


■'  ; 


i-..-  <.. 


M 


l:;i 


8 


IMPROVEMENTS  IN  ASTRONOMICAL  INSTRUMENTS. 


this  ingenious  plan  will  work.  It  will  certainly  have  the  advantage 
that  a  slow  motion  can  be  given  with  less  expenditure  of  power  than 
on  any  other  system ;  but  if  the  motion  is  at  all  rapid  I  am  inclined  to 
suspect  that  the  friction  of  the  fluid  will  be  equal  to  that  of  the  rollers 
on  the  other  system.  The  difficulty  which  I  should  principally  fear  is 
the  leaking  of  the  caisson.  The  freezing  of  the  water  will  be  avoided 
by  impregnating  it  with  chloride  of  magnesium.  It  is  intended  to  con- 
struct a  dome  on  this  principle  20  meters  in  diameter  for  the  great  re- 
fracting telescope  now  being  constructed.  It  is  feared,  however,  that 
the  practical  completion  of  the  work  will  be  long  postponed,  owing  to 
the  necessity  of  finding  a  better  foundation  for  the  structure  than  is  now 
afforded  by  the  grounds  of  the  observatory. 

THE  GREAT  BUSSUN  TELESCOPE. 

In  1879  Privy  Counselor  Otto  Von  Struve,  director  of  the  Pulkowa 
Observatory,  visited  this  country  and  contracted  with  the  Messrs.  Clark 
for  the  construction  of  an  objective  30  inches  in  aperture.  It  was  com- 
pieced  and  delivered  during  the  year  1882.  The  mounting  is  now  being 
completed  by  the  Messrs.  Bepsold,  of  Hamburg.  Although  still  nnfln- 
ished,  I  was  desirous  of  gaining  all  the  information  possible  respecting 
its  construction,  and  therefore  visited  Hamburg  for  the  puri>08e  of  ex- 
amining its  parts.  The  following  are  some  essential  points  in  the  struc- 
ture: 

The  most  striking  feature  of  the  instrument  will  be  the  absence  of 
fHction  rollers  from  the  declination  axis.  With  so  large  an  instniment 
the  firiction  on  the  declination  axis  will  be  too  great  to  admit  of  the  tele- 
scope being  conveniently  turned  either  by  hand  or  by  a  rope  attached 
to  the  two  ends,  as  at  Washington  and  Vienna.  The  quick  motion  in 
declination  will  be  given  by  a  system  of  cog-wheels  turned  by  an  axis 
passing  through  the  polar  axis  of  the  instrument  and  coincident  with 
it.  This  axis  will  be  turned  by  a  crank  at  the  lower  end,  or  by  the  ob- 
server taking  hold  of  the  circumference  of  a  wheel,  at  choice.  Although 
the  turning  of  the  crank  is  a  more  convenient  motion  for  the  purpose 
than  that  of  taking  hold  of  the  handles  of  a  steering-wheel,  I  do  not 
consider  it  so  convenient  as  pulling  a  rope.  This  system  of  wheel- work 
will  also  be  connected  with  the  axis  of  a  crank  at  the  eye-piece  which 
the  observer  can  take  hold  of  and  turn  without  leaving  the  eye-end  of 
the  telescope.  A  second  crank  will  be  furnished  for  the  motion  in  right 
ascension. 


5TRUMENTS. 

have  the  advantage 
diture  of  power  than 
apid  I  am  inclined  to 
to  that  of  the  rollers 
aid  principally  fear  is 
trater  will  be  avoided 
It  is  intended  to  con- 
Bter  for  the  great  re- 
feared,  however,  that 

postponed,  owing  to 
structure  than  is  now 


DPB. 

•ector  of  the  Pulkowa 
with  the  Messrs.  Clark 
bpertnre.  It  was  com- 
mounting  is  now  being 
Although  still  nnfln- 
9n  possible  respecting 
•  for  the  puriK)se  of  ex- 
tial  points  in  the  strnc- 

will  be  the  absence  of 
90  large  an  instniment 
Mit  to  admit  of  the  tele- 
or  by  a  rope  attached 
.  The  quick  motion  in 
eels  turned  by  an  axis 
it  and  coincident  with 
)wer  end,  or  by  the  ob- 
)l,  at  choice.  Although 
notion  for  the  purpose 
leering- wheel,  I  do  not 
is  system  of  wheel- work 
at  the  eye-piece  which 
leaving  the  eye-end  of 
for  the  motion  in  right 


IMPROVEMENTS    IN   ASTRONOMICAL    INSTRUMENTS.  9 

Instead  of  using  a  sector  for  the  clock  motion  the  screw  will  gear  into 
a  complete  wheel  about  two  meters  in  diameter.  The  trouble  of  having 
to  turn  the  sector  back  will  thus  be  avoided.  The  illumination  of  the 
finding  circles  and  the  arrangements  for  reading  them  will,  in  their 
results,  be  similar  to  those  used  on  other  large  telescopes:  that  is,  the 
arrangement  will  be  such  that  the  observer  can  read  either  circle  from 
the  eye-piece.  The  system  of  illuminating  the  field  wires,  micrometer, 
position  circle,  &c.,  though  extensively  employed  in  Europe,  is  so  little 
known  in  this  country  that  attention  should  be  called  to  it.  The  side 
of  the  telescope  at  a  convenient  distance  above  the  eye-end  is  pierced 
by  an  opening  on  the  opposite  side  from  the  declination  axis.  Through 
t&is  opening  passes  a  conical  tube  parallel  to  the  declination  axis.  At 
the  outer  end  of  this  tube  is  a  reflector  inclined  at  an  angle  of  45  degrees 
to  the  axis  of  the  cone,  but  turning-  on  an  axis  coincident  with  that  of 
the  cone.  The  illuminating  lamp  shines  upon  this  reflector  and  turns 
upon  the  same  axis  with  it.  It  is  also  hung  upon  gimbals  so  as  to  turn 
upon  a  secondary  axis  coincident  with  the  axis  of  its  own  line  of  light. 
The  resnlt  of  this  arrangement  is  that  the  lamp  always  hangs  vertically, 
whatever  the  position  of  the  telescope,  and  that  the  horizontal  beam  of 
rays  thrown  ftom  it  always  strikes  the  mirror  at  an  angle  of  45  de^^rees 
in  such  a  way  as  to  throw  the  light  directly  through  the  conical  tube 
and  into  the  telescope. 

The  slightly  divergent  beam  which  fills  the  cone  is  divided  into  two 
or  three  concentric  portions.  One  of  these  is  reflected  upward  to  the 
object-glass,  and  by  reflection  firmn  the  glass  itself  illuminates  the  field 
of  view.  Another  portion  shines  upon  four  whitened  surfaces  around 
the  sides  of  the  micrometer,  by  which  both  sets  of  wires  are  illuminated. 
The  portion  of  light  which  is  not  needed  for  this  purpose  is  so  arranged 
as  to  illuminate  the  two  verniers  of  the  position  circle  and  the  heads  of 
the  micrometer.  So  far  as  I  could  judge,  the  working  of  this  plan  leaves 
nothiL„'  to  be  desired  in  the  way  of  convenience  to  the  observer. 

Worthy  of  special  attention  are  the>  eye-piece  micrometers  now  made 
by  the  Messrs.  Bepsold.  They  include  every  contrivance  necessary  for 
rapid  and  convenient  use. 

Support  of  the  polar  axis. — Another  important  feature,  which  has  been 
applied  by  the  Bepsolds  in  their  other  large  instruments,  is  the  method 
of  supporting  the  iralar  axis.  This  axis  has  to  bear  a  large  part  of  the 
instrnment,  counterpoises  included.  Asjordinarily  made,  it  is  necessa- 
rily snbleot  to  an  end  thrust  equal,  in  our  latitude,  to  two-thirds  the 


,.^;;M*r«W«' 


^"aWirflVi-ilr' 


•      V 


10 


IMPROVEMENTS   IN   ASTRONOMICAL   INSTRUMENTS. 


weight  of  the  instrument.  How  to  support  this  thrust  without  inter- 
fering with  the  ease  and  freedom  of  motion  has  been  one  of  the  difficult 
problems  in  mounting  a  telescope.  In  the  Bepsold  instrument  the 
thrust  is  nearly  avoided  by  supporting  the  polar  axis  upon  a  vertical 
friction -wheel  under  the  center  of  gravity  of  the  entire  instrument. 
Oounterpoises  can  be  placed  at  the  lower  end  of  the  axis  so  as  to  bal- 
ance the  instrument  upon  this  wheel.  So  far  as  I  can  judge,  this  plan 
leaves  nothing  to  be  desired. 

PBACTIOAL  OONOLUSION8. 

I  have  been  led  by  the  examination  above  described,  combined  with 
some  experience  in  the  use  of  the  Washington  telescope,  to  some  con- 
clusions resiiecting  the  most  appropriate  features  in  the  mounting  of  an 
instrument  of  the  larger  size.  They  may  be  here  enumerated  for  the 
consideration  of  those  engaged  in  cbnstnictions  of  this  kind. 

I.  I  think  that  in  order  to  secure  the  necessary  stiffness  with  the 
least  weight  the  axes  should  be  hollow.  The  material  can  then  be  made 
comparatively  thin.  It  is  true  that  the  larger  the  axis  the  greater  the 
friction.  But  the  mass  of  metal  in  the  interior  of  the  axis  contributes 
so  little  to  its  stiffness  that  the  external  diameter  will  have  to  be  in- 
creased very  little  to  secure  the  same  stiffness  with  the  hollow  axis  as 
with  the  solid  one. 

II.  It  is  not  worth  while  to  supply  the  declination  axis  with  friction 
rollers  unless  experiment  and  research  shall  show  that  they  can  be  made 
more  effective  than  they  appear  to  be  in  the  Vienna  instrument 

III.  The  best  quick  motion  in  right  ascension  is  that  adopted  in  the 
Washington  telescope,  where  the  observer  i)ulls  an  endless  rope  hand 
over  hand,  and  can  lock  and  unlock  the  gearing  which  connects  the 
turning-wheel  with  the  telescope  at  pleasure. 

lY.  If,  as  is  possible,  the  quick  motion  in  declination,  by  means  of  a 
loose  rope  attached  to  the  two  ends  of  the  telescope,  requires  too  strong 
a  pull,  the  best  method  of  giving  this  motion  is  through  a  gearing  turned 
by  an  axis  passing  centrally  through  the  polar  axis  on  the  Bepsold 
plan.  But  it  is  preferable  to  have  this  motion  made  by  turning  a  crank 
or  pulling  a  rope  rather  than  by  taking  hold  of  a  wheel. 

V.  Coarse  divided  wheels  should  be  supplied,  so  that  the  observer 
while  turning  the  instrument  can  constantly  see  its  approximate  point- 
ing. It  is  better  if  this  coarse  reading  can  be  made  with  the  naked  eye, 
as  is  the  case  In  the  right  ascension  movement  of  the  Washington  tele- 


TRUMENT8. 

bhrast  without  inter- 
in  one  of  the  difficult 
sold  instrument  the 
axis  upon  a  vertical 
e  entire  instrument, 
the  axis  so  as  to  bat - 
can  judge,  this  plan 


ibed,  combined  with 
Bscope,  to  some  con- 
1  the  mounting  of  an 

enumerated  for  the 
;his  kind. 

y  stiffness  with  the 
ial  can  then  be  made 

axis  the  greater  the 
the  axis  contributes 
'  will  have  to  be  in- 
h  the  hollow  axis  as 

on  axis  with  fHotion 
lat  they  can  be  made 
a  instrument 
that  adopted  in  the 
a  endless  rope  hand 
which  connects  the 

ition,  by  means  of  a 
),  requires  too  strong 
igh  a  gearing  turned 
bxis  on  the  Bepsold 
B  by  turning  a  crank 
rheel. 

10  that  the  observer 
B  approximate  point- 
)  with  the  naked  eye, 
he  Washington  tele- 


IMPROVEMENTS  IN  ASTRONOMICAL  INSTRUMENTS. 


11 


scope.  The  declination  circle  being  farther  from  the  observer,  it  has  to 
be  read  with  an  opera-glass  if  more  than  a  coarse  fraction  of  a  degree 
is  required.  By  such  an  arrangement  the  telescope  can  always  be  set 
by  the  quick  motion  so  nearly  that  any  object  sought  shall  be  in  the 
field  of  view  of  the  finder.  In  nine  cases  out  of  ten  this  will  be  all  that 
is  required  in  practical  use.  It  should  never  be  forgotten  that  in  all 
quick  motions  it  is  very  desirable  that  the  observer  shall  be  able  to  keep 
his  eye  upon  the  movements  of  the  telescope  itself  in  order  to  save  him 
firom  any  apprehension,  even  a  groundless  one,  that  something  may  be 
going  wrong. 

VI.  The  slow  motion  should  if  possible  be  endless.  There  is  no  dif- 
ficulty in  making  it  so  in  right  ascension ;  though  there  may  be  in  dec- 
lination. 

VII.  When  the  instrument  is  so  large  that  there  is  an  interval  of  three 
feet  or  more  between  the  center  of  the  polar  axis  and  the  side  of  the 
tube,  the  screw  which  communicates  the  clock  movement  should  be 
geared  into  a  complete  circle  rather  than  into  a  sector.  The  use  of  the 
metal  band  to  multiply  the  effective  radius  of  the  wheel  offers  no  advan- 
tage in  the  case  of  large  instruments  to  compensate  for  the  disadvan- 
tage of  want  of  stability  arising  from  elasticity  of  the  band  and  its  fast- 
enings. 

VIII.  In  this  connection  should  be  considered  the  question  of  apply- 
ign  the  system  of  Aing,  which  consists  in  giving  a  dock-motion  to  the 
verniers  of  the  right-ascension  circle  so  that  their  position  shall  repre- 
sent sidereal  time.  Every  practical  astronomer  is  familiar  with  the 
trouble  in  setting  an  ordinary  equatorial,  arising  firom  the  necessity  of 
having  to  calculate  the  constantly  varying  hour  angle  of  the  object  on 
which  he  points.  With  the  Greenwich  arrangement  there  is  no  such 
trouble.  The  damping-wheel  being  once  set  to  sidereal  time,  the  ob- 
server has  only  to  set  the  other  one  to  the  constant  right  ascension  of 
the  object.  It  is  true  that  practical  difficulty  arises  in  the  usual  con- 
struction, owing  to  the  fact  that  the  vernier  on  the  gear-wheel  will  from 
time  to  time  be  on  every  point  of  tfa  circle.  But  this  difficulty  can,  I 
think,  be  obviated  by  appropriate  arrangements. 

IX.  A  dock  motion  which  can  be  kept  up  by  water  or  other  power  is 
greatly  preferable  to  any  system  which  requires  an  assistant  to  wind  up 
a  weight. 

X.  The  entire  practicability  of  illuminating  the  divisions  of  the  circles 
by  a  lamp  and  of  reading  these  divisions  firom  the  eye-end  of  the  telescope 


m0iitiiimimmm»mtitm 


4^  I 


1  ^        - 

ii 

/*."'  '^^J^i ' 

'^^^'^f^l 

Is^^t^^ 

B 

12 


IMPBOVEMENTS    IN    ASTRONOMICAL    INSTRUMENTS. 


has  been  so  completely  demonstrated  that  all  large  instruments  should 
be  supplied  with  this  arrangement.  It  can  hardly  be  doubted  that  elec- 
tric lights  will  hereafter  take  the  place  of  lamps  for  this  purpose. 

XI.  The  systeVn  of  illuminating  wires,  field,  micrometer-head,  &c.,  by 
a  single  lamp,  which  shall  be  vertical  in  all  positions,  has  been  so  per- 
fected by  the  Eepsolds  that  it  leaves  nothing  to  be  desired. 

XII.  The  Washington  plan  of  having  the  whole  micrometer  plate,  in- 
cluding both  fixed  and  movable  wires,  moved  by  a  fine  screw  which  has 
not  necessarily^  a  divided  head,  offers  such  a  convenience  in  setting  that 
it  should  always  be  adopted. 

XIII.  The  old  system  of  having  a  single  finder  on  that  side  of  the 
telescope  which  is  opposite  the  declination  axis  becomes  very  inconven- 
ient in  a  large  i^nstrument,  owing  to  the  necessity  of  setting  the  slit  in 
the  dome  not  only  to  the  telescope  but  to  the  finder.  The  plan  adopted 
in  the  Vienna  telescope  of  having  two  finders,  of  which  one  shall  be  above 
and  the  other  below  the  telescope  when  the  latter  is  in  the  meridian, 
obviates  this  difficulty  and  should  always  be  adopted. 

BEFLECTINO  TELESCOPES  IN  FRANCE. 

It  is  well  known  to  all  who  have  given  attention  to  this  subject  that 
the  optical  pertbrmance  of  great  reflecting  telescopes  has  never  been 
proportional  to  their  size,  and  that  the  mechanical  difficulties  of  keeping 
a  large  reflector  in  proper  figure  in  different  positions  have  been  appar- 
ently insurmountable.  A  plan  of  supporting  a  large  mirror,  devised  by 
the  Messrs.  Henry,  has  been  adopted  in  Paris,  which  it  is  hoped  may  ob- 
viate this  difficulty.  It  consists,  in  principle,  in  supporting  the  mirror 
upon  a  mass  of  metal  of  a  form  similar  to  that  of  the  mirror,  the  surfiEkoe 
of  which  is  ground  to  fit  the  lower  surface  of  the  mirror  with  accuracy 
when  the  latter  is  in  proper  shape.  If  the  mirror  rested  directly  in 
contact  with  this  second  si^rface  no  advantage  would  be  gained,  since 
the  backing  itself  would  bend  as  readily  as  the  mirror.  Therefore  be- 
tween the  two  is  inserted  a  thin  stratum  of  some  elastic  substanqe.  M. 
Henry  has  found  a  sheet  of  fine  flannel  to  give  the  best  results.  The 
effect  of  the  sheet  is  to  diminish  the  flexure  of  the  mirror  by  a  fraction 
depending  upon  its  stiffness  and  upon  the  elasticity  of  the  flannel. 
Theoretically  it  may  be  considered  imperfect,  because,  in  order  to  act, 
some  stiffhess  is  required  in  the  mirror  itself.  A  perfectly  flexible  mir- 
ror would  bend  just  as  much  with  the  flannel  as  without  it  But  flex- 
ure of  the  mirror  can,  it  appears  to  me,  be  reduced  to  quite  a  small 


■MIlMMMMMibB 


^STRUMENTS. 

ge  instrumeuts  should 
y  be  doubted  that  elec- 
for  this  purpose, 
croiueter-head,  &c.,  by 
tious,  has  been  so  per- 
be  desired. 

e  micrometer  plate,  in- 
a  fine  screw  which  has 
enieuce  in  setting  that 

ler  on  that  side  of  the 
tecomes  very  inconven- 
Dy  of  setting  the  slit  la 
ler.  The  plan  adopted 
<rhich  one  shall  be  above 
ter  is  in  the  meridian, 
ipted. 

BANOB. 

ion  to  this  subject  that 
iscopes  has  never  been 
h\  difficulties  of  keeping 
itions  have  been  appar- 
arge  mirror,  devised  by 
'hich  it  is  hoped  may  ob- 
I  supporting  the  mirror 
I  the  mirror,  the  snrfiEMM 
ae  mirror  with  accuracy 
lirror  rested  directly  in 
would  be  gained,  since 
)  mirror.    Therefore  be- 
i  elastic  substance.    M. 
)  the  best  results.    The 
the  mirror  by  a  fhution 
lastioity  of  the  flannel, 
ecause,  in  order  to  act, 
A.  perfectly  flexible  mir- 
8  without  it    Bat  flez- 
■educed  to  quite  a  small 


IMPROVEMENTS   IN   ASTRONOMICAL   INSTRUMENTS. 


13 


fraction  of  its  amount.  Moreover,  I  see  no  insuperable  objection  to  the 
superposition  of  two  systems  of  the  kind,  the  mirror  resting  upon  a  stiff 
disk  which  is  itself  supported  upon  a  second  one. 

This  plan  has  beon  entirely  successful  in  the  cases  in  which  it  has 
been  applied,  mirrors  up  to  12  inches  in  diameter  showing  not  the  slight- 
est flexure  when  moved  into  all  practical  positions.  Unfortunately  it 
has  not  yet  been  tried  with  reflectors  of  a  larger  size. 

THE  EQUATORIAL  COUD^. 

I 

By  applying  the  simple  method  just  described  for  mounting  mirrors, 
an  equatorial  surpassing  all  others  in  convenience  of  use  has  been  put 
into  operation  at  the  Paris  Observatory.  The  plan  of  having  a  telescope 
of  which  the  tube  itself  should  be  the  polar  axis,  so  that  the  eye-piece 
should  constantly  point  towards  the  north  pole,  is  quite  familiar  to  as- 
tronomers.   The  plan  heretofore  proposed  is  this: 

Below  the  object-glass,  which  in  the  northern  hemisphere  would  point 
towards  the  south  pole,  is  to  be  placed  a  reflector  capable  of  turning 
round  an  axis  at  right  angles  to  that  of  the  telescope  while  the  latter 
turns  upon  its  own  axis.  The  latter  motion  would  the  measure  right 
ascension  and  the  revolution  of  the  mirror  would  be  one-half  the  change 
of  the  declination.  This  plan  is  subject  to  the  inconvenience  that,  in 
order  to  look  near  the  south  horizon,  the  mirror  would  have  to  be  much 
elongated,  while  the  view  around  the  north  pole  would  always  be  cut  off 
by  the  intervention  of  the  telescope  itself. 

In  the  equatorial  ooud^  this  difficulty  is  obviated  by  the  use  of  a  sec- 
ond reflector.  The  telescope,  as  its  name  indicates,  is  elbow-shaped. 
Its  lower  part  consists  of  an  arm  at  right  angles  to  the  axis.  At  the  el- 
bow is  fixed  a  mirror,  from  which  the  light  is  reflected  at  an  angle  of  45 
degrees.  At  the  outer  end  of  the  arm  is  a  second  reflector,  also  at  an 
angle  of  45  degrees,  and  turning  upon  a  central  axis  of  this  arm.  By  its 
motion,  the  field  of  view  of  the  telescope  will  sweep  over  a  belt  of  uni- 
form width  from  the  pole  to  the  horizon,  so  that  its  position  angle  will 
correspond  to  declination.  By  turning  the  whole  instrument  on  its  axis 
the  field  of  view  will  sweep  through  a  zone  of  constant  declination.  The 
object-glass  is  in  the  arm  between  the  two  reflectors.  The  angle  of  re- 
flection firom  each  mirror  is  constantly  45  degrees. 

The  advantage  of  this  construction  is,  that  the  observer  does  not  have 
to  follow  the eje-pieoeof  his  telescope,  bat  always  sits  in  a  fixed  posi- 
tion in  a  comfortable  room.    All  the  motions  and  all  the  readings  are 


IMPKOVEMENTS  IN  ARTRONOMICAL  INSTRUMENTS. 

made  as  be  sits  at  the  eye-piece.  The  amount  of  work  of  a  certain  clans 
that  can  be  done  with  the  telescope  is  thns  greatly  increased.  The  form 
is,  however,  inadmissible  in  au  instrument  in  which  the  highest  optical 
power  is  aimed  at,  owing  to  the  loss  of  light  by  the  double  retieotion. 
In  a  large  instniment,  I  should  also  fear  injury  to  the  images  fh>m  the 
bending  of  the  mirror,  but  no  such  effect  shows  itself,  at  least  in  any 
striking  degree,  in  the  Paris  instrument,  which  is  of  about  ten  English 
inches  aperture. 

THE  BTRABSBUBO  MKBIDIAM  OIBOLB. 

This  instrument  is  commonly  considered  to  embody  the  latest  concep- 
tions in  astronomical  mechanics.  Its  general  design  is  founded  on  that 
adopted  in  the  great  meridian  circle  of  the  Harvard  College  Observ- 
atory which  was  constructed  by  Troughton  and  Simms,  of  London.  The 
original  design  of  the  latter  instrument  is,  it  is  understood,  largely  due 
to  the  late  Professor  Winlock.  The  most  essential  modification  of  the 
older  plan  is  that  the  T's  and  the  reading  microscopes,  instead  of  being 
supported  upon  piers  of  stone,  are  borne  by  a  massive  metal  foundation, 
the  tops  of  the  piers  being  below  the  level  of  the  bottoms  of  the  circles. 
The  drawbacks  arising  fh>m  the  unequal  contraction  and  expansion  of 
the  stone  piers,  under  the  influence  of  variations  of  temperature,  are  thns 
almost  entirely  avoided,  because  the  metallic  supports  rapidly  assume 
the  temperature  of  the  surrounding  air  and  of  the  instrument. 

Every  part  of  the  instniment  bears  the  impress  of  the  thought  and 
care  devoted  to  its  construction  both  by  the  makers  (the  Messrs.  Bepsold) 
and  by  Professor  Winnecke,  the  director  of  the  observatory.  Even  the 
form  of  the  piers  of  masonry  which  support  it  and  its  collimators  is 
highly  original.  The  base  of  the  principal  pier  is  smaller  than  usna!, 
an^  the  amount  of  material  in  it  is  still  farther  diminished  by  building 
it  in  the  form  of  a  Greek  cross.  The  collimators  are  supported  on  cy- 
lindrical piers  of  the  usual  oonstmotion.  Each  of  these  three  piers  is 
protected  from  changes  of  temperature  by  having  a  hollow  cylinder  of 
brick  built  up  around  it  from  the  ground.  The  thickness  of  the  wall  of 
this  cylinder  is  that  of  one  brick.  To  insure  stability  the  difBarent  cyl- 
inders are  connected  together  by  brick  arches,  but  these  arches  do  not 
exert  any  pressure  upon  the  interior  piers  supporting  the  instrument, 
which  rests  only  on  their  foundation.  A  degree  of  stability  is  thns 
secured  which  I  believe  has  never  before  been  reached. 

But  this  may  be  in  great  part  due  to  the  excellence  of  tlie  foundation. 


<8TRUMENT8. 

work  of  a  certain  oUms 
f  increased.  The  form 
ich  the  highest  optical 
the  double  reflection. 
9  the  images  from  the 
itself,  at  least  in  any 
I  of  about  ten  English 

BOLB. 

K>dy  the  latest  concep- 
lign  is  founded  on  that 
'vard  College  Observ- 
nms,  of  London.  The 
nderstood,  largely  due 
al  modifloation  of  the 
sopes,  instead  of  being 
sive  metal  foundation, 
bottoms  of  the  circles. 
Jon  and  expansion  of 

temperature,  are  thus 
}port8  rapidly  assume 
e  instrument. 
BS  of  the  thought  and 
}  (the  Messrs.  Repsold) 
>servatory.  Even  the 
and  its  collimators  is 
is  smaller  than  usual, 
iminished  by  building 

are  supported  on  cy- 
of  these  three  piers  is 
g  a  hollow  cylinder  of 
lickness  of  the  wall  of 
ility  the  different  cyl- 
it  these  arches  do  not 
>rting  the  instrument, 
le  of  stability  is  thus 
Mshed. 
nee  of  tlie  foundation. 


IMPROVEMENTS   IN    ASTRONOMICAL   INSTRUMENTS. 


15 


The  observatory  building  rests  u^n  a  stratum  of  gravel  so  clean  and 
pure  that  in  case  of  a  flood  in  the  Rhine  the  water  permeates  through 
the  gravel  to  the  base  of  the  piers.  It  might  be  supposed  that  water 
thus  penetrating  the  foundation  would  produce  an  injurious  effect  upon 
the  stability,  but  such  is  not  found  to  be  the  case.  The  fact  that  gravel 
forms  the  best  foundation  for  an  astronomical  instimment  has  long  been 
understood  by  those  who  have  given  attention  to  the  subject.  But  I 
do  not  know  of  any  other  case  in  which  the  saturation  of  the  gravel 
with  water  has  been  experienced. 

I  may  mention  in  this  connection  that  a  solid  bed-rock  might  be  even 
better  than  gravel  were  it  covered  with  so  deep  a  layer  of  soft  earth 
that  ii  would  not  be  affected  by  daily  or  annual  changes  of  temperature. 
Experience  has,  however,  shown  that  for  want  of  these  conditions  being 
llilfllled  a  solid  rock  forms  a  very  unsafe  foundation.  An  interesting 
example  of  this  is  afforded  by  the  observatory  at  Neuchatel,  which  is 
erected  at  the  base  of  the  Jura  Mountains.  The  annual  change  in  the ' 
pointing  of  the  meridian  circle  is  so  great  that  Dr.  Hirsch  has  recently 
published  an  investigation  of  the  subject,  showing  that  the  mountain 
undergoes  an  annual  change  to  an  extent  which  has  never  before  been 
remarked. 

In  order  to  obtain  an  accurate  estimate  of  the  stability  of  the  Strass* 
burg  instrument,  I  requested  the  acting  director.  Dr.  Schur,  to  allow  me 
to  transcribe  the  instrumental  oorrectfons  during  as  long  a  period  as 
practicable.  The  following  are  the  values  of  the  three  instrumental  con- 
stants which  depend  on  the  deviation  of  the  axis  of  rotation  from  a  true 
east  and  west  line.  Column  •  gives  the  level  correction ;  n,  the  distance 
of  the  line  of  collimation  east  of  the  pole;  m  the  deviation  of  the  same 
line  at  its  point  of  intersection  with  the  equator.  Of  these  constants  n 
is  more  accurately  determined  than  either  of  the  others,  and  its  stability 
affords  a  test  of  the  stability  of  the  instrument  both  in  level  and  admnth. 


i. 

n. 

M. 

1888. 
Jane  17 
Jnly     3 
10 
14 
15 
17 
80 
87 

1. 

.00 
-.08 
-.03 

: 

+.03 
-.02 
+.06 
+.01 

+.11 
+.03 
+.06 
+.11 

«. 

-.03 
-.08 
-.11 

+.01 

+.06 

.00 

-.01 
+.06 
-.18 

.?.  .'' 


19 


IMPBOV£MENTS    IN   A8TH0N0MICAL   INSTRUMENTS. 


Sept, 


Oct. 


1889. 

Aug.  34 

i<5 

89 

1 

It 

6 

V 

Itf 

35 

85 

4 

7 

13 

17 

19 

81 

8» 

86 

88 

30 

31 

1 

1 

3 

6 


(. 
+.13 
+.89 
+.09 
+.06 
+.01 
03 
08 
01 
01 
01 
83 


I 


Not. 


loo 

+.08 
+.08 
.00 
+.01 
+.06 
—.01 
+.01 
-■.04 
--.03 
--.05 
-}-.01 
-..00 
--.03 


f    *• 

m. 

*. 

f. 

+.18 

-.01 

+.06 

+.04 

+.UJ 

—.11 

.00 

+.01 

+.09 

-.06 

+.80 

-.19 

+.08 

—.08 

+.02 

+.01 

-.01 

.00 

+.07 

+.96 

+.05 

-.05 

-.05 

--.18 

-.04 

--.17 

-..06 

—.06 

--.04 

-.03 

—.02 

+.13 

+.08 

-.10 

—.03 

+.04 

-.18 
-■.05 

-.08 

-.01 

...06 

+.03 

--.01 

.00 

-..06 

-.06 

+.08 

—.08 

It  will  be  seen  that  if  we  take  the  mean  value  of  n,  0*.06,  as  a  constant 
for  the  four  months  the  average  deviation  of  the  observed  values  from 
this  mean  will  be  less  than  0*.06.  A  portion  of  these  deviations  are  due 
to  errors  of  determination  and  the  accidental  deviations  of  a  temporary 
character  due  to  changes  in  th» temperature  of  the  different  parts  of  the 
instrument,  produced  by  the  impacts  of  air  currents. 

The  stability  of  the  nadir  point  appeared  much  less  satisfactory. 
This  correction  was  not  determined  with  sufficient  regularity  to  admit 
of  its  changes  being  the  subject  of  a  positive  calculation,  and  Dr.  Sohur 
expressed  himself  not  entirely  satisfied  of  the  accuracy  of  the  determi- 
lyitions.  It  seems  scarcely  possible  that  the  polar  pointing  of  the  in- 
stmment  could  be  so  steady  if  the  nadir  point  were  subject  to  consider- 
able changes. 

The  stone  piers  are  terminated  on  top  by  horizontal  faces,  in  which 
are  set  the  two  iron  supports  of  the  T's  and  of  the  microscopes.  The 
base  of  each  is  an  iron  frame  18  inches  from  east  to  west,  by  24  inches 
flrom  north  to  south,  which  is  set  on  the  top  of  the  stone  and  held  in  place 
by  iMing  bedded  in  cement.  Upon  this  sets  the  base  of  the  microscope 
bolder,  which  rests  on  three  feet,  and  may  be  adjusted  horisontally  by 
screws.    I  should  myself  suppose  that  greater  security  against  aooi. 


■"•MaHUMMMi 


rSTRUMENTS. 


/ 


-.01 

f.64 
-.11 

-.06 
-.19 
-.08 
■f-.OJ 

.00 
■f.SW 
-.05 
+.18 
+.17 
—.06 
—.03 
+.13 
-.10 
+.04 
-.08 
-.01 
+.03 

.00 
-.06 
—.08 


of  n,  0».06,  as  a  constant 
le  observed  values  from 
bhese  deviations  are  due 
iviations  of  a  temporary 
he  different  parts  of  the 
■ents. 

much  less  satisfactory, 
ient  regularity  to  admit 
loulation,  and  Dr.  Sohur 
iccuraoy  of  the  determi- 
)lar  pointing  of  the  in- 
rere  subject  to  consider- 

orizontal  faces,  in  which 
r  the  microscopes.  The 
ast  to  west,  by  24  inches 
le  stone  and  held  in  place 
le  base  of  the  microscope 
acyusted  horisontally  by 
Br  security  against  aoci- 


IMPROVEMENTS  IN  ASTRONOMICAL  INSTRUMENTS. 


17 


dental  change  would  have  been  secured  if  the  top  face  of  the  frame  had 
each  been  planed  to  lit  the  base  of  the  holder  instead  of  having  the  latter 
on  feet,  provided  some  side  support  were  added  to  guard  against  any 
possible  minute  rocking  motion. 

The  mioroitcopes  are  carried  on  the  external  surface  of  a  cylinder, 
which  is  oast  in  the  same  piece  with  the  base  last  described. 

The  fhlorum  on  whioh  rests  each  lever  supporting  the  weights  of  the 
instrument  at  one  end  and  the  counterpoise  at  the  other  is  itself  sup- 
imrted  upon  the  cylinder  carrying  the  microeoopes.  This  feature  of  the 
iu8t.niment  has  been  objected  to  on  the  ground  that  the  mioroscoiie 
caniers  should  not  be  sab|}eeted  to  so  great  a  pressure,  which  may. pos- 
sibly b«  subject  to  vibratory  changes  as  the  instrument  tnrns  upon  the 
friction  rollers.  Although  it  is  quite  possible  that  no  actual  evil  results 
from  this  cause,  it  must  yet  be  regarded  as  a  not  improbable  source  ot 
danger  to  the  steadiness  of  the  microscope  holders ;  I  therefore  con- 
sider that  it  would  be  better  to  adopt  some  other  system  of  support- 
ing the  instramait)  perhaps  upon  i^btfs  passing  centrally  through  the 
microscope  holders  and  set  in  the  pier  below. 

The  arrangauieiits  far  okmpiiig  the  miforosoopes  to  the  cylinder  leave 
nothtef :  to  b«  deeteed,  provided  tJie  fwrner  ate  once  got  into  their  pwper 
poaiMon.  Butth«ta«kofsetlUig«miflra«eppe«ftwitisonoediaarranged 
is  •atratawlir  laboiioas,  aud  aoBie  addittonal  meehanism  fi»r  eflboting  Hiis 
isxlMterillew 

<nM.ftecnnM9rjrWi>w4M0htlM'divisione  ea  ,tiieeiiole«recttt,aiid'the 
adaplaiisu  ofiMn  AUoroeospe  to  Ihfsir  reading,  are  nadonbtodlar  Ibe 
gwtort  ipaiaite  afidilioallit  in  «ks  aoMteaolion^a  iMfktA.  oind*  Af >  Die 
first  order.  I  am  oonvinoed  that  muoh  must  yet  be  done  to  •eoBn^tke 
beat KMuMs  in  tUft teapeM.  Ls  tM'Btg—liiwg  oiral%  aail  In  th*otlier 
lee— jilustMirtsof  th»  BiienliiS)itha;diaB|Btog(iif  theaiiol^ia  isdutwd 
to  twoAal.  .<Oue  gwat  <Miyantog»ia.ttaa aaanridtin Jhat^lhe tfintoaJi 
ti|»juionNeope  aN'lMWMil)^!  to^|arioqaohMifle*imai  ettrr^Hstof  air 
iif  ilMMul  trtuiMiiiliiin  BiyliadnMrlMM^iaafethe  «MM«iaM>iiiftro- 
diMiiitoaUlM  iMfethafc,  a»tlH»-«iaiiietoi!  Ht^liufaiiahrt^— y  msm,ai,a, 
«lwp  Miouit  i«  iiMp«Miti«fc«f  .^.division  <will'|W0d9afta  pra|HttitiB- 
aUy  4gnatoii  jeJBMfe  in  taading  ag  Iha  aafie.  fienoe,  li»  order  that  no 
aenBMiyiaiayjlwJoatJtoaftiaaai^M^liWiahsolatettrotaf  thadteiMaaa, 
thd«ahMTi«aa,  and  lha.p<wwgttfitha.mi<wi(Mepaa,  maat  all  be  inanaied 
in  Aa  aipMigreiaitiiDa  «ha*  >tka>diaaeCcr  af  <<iba.einl».ia  dimiaialMd. 
t8»  IfaciW?-^-^ 


im'. 


II 


IMPROVEMEKTS   IN   A8TH0N0MICAL   INSTRUMKNTH. 


P  I 


^■f'i 


'■•■'•         i 

V 

'■■■•V  ■;■■ ' 


It  do«fl  not  appear  to  me  that  the  eminent  constrnctors  of  the  inatrn- 
ment  have  ancceedeil  in  this.  In  onler  that  I  might  reach  a  precise 
couclaaion  on  thia  point,  I  aaked  iiermiaaion  at  Btrasabarg  to  collect  the 
data  and  to  make  the  determination  necessary  for  a  rough  oomparisou 
with  the  Washington  circle. 

First,  as  regards  errors  of  division,  I  found  that  these  errors  bad  l>eeii 
determined  for  every  5  degrees.  The  best  method  of  making  a  numeri- 
cal estimate  of  the  accidental  errors  of  division  seems  to  be  to  compare 
the  error  of  each  division  with  the  mean  of  the  errors  of  the  two  aton- 
ing onett  differing  by  6  degrees.  I  examined  the  table  of  errors  through  - 
a  portion  of  the  circle  with  the  result  that  the  mean  error  as  thus  de- 
termined is  0''.32,  while  the  maximum  is  0".63.  It  will  be  interesting  to 
compare  this  with  the  Washington  circle. 

On  page  37  of  the  Washington  Observations  for  1860  is  found  a  table 
of  the  errors  of  the  two  circles  of  the  Washington  instrument.  Treat- 
ing them  in  the  same  way,  we  find: 

Waabington,  circle  A:  mean  error,  (K'.28;  maximum,  W.79. 

Washington,  circle  B:  mean  error,  0".21;  maximum,  0".31. 

Straasbnrg  circle:  mean  error,  <K'.S3;  maximum,  (K'.63. 

It  will  be  seen  that  in  angular  position  the  errors  of  tlie^BtraMborg 
circle  are  in  general  about  the  same  as  those  of  circle  A  of  the  WMh- 
Ington  instrument,  but  are  decidedly  inferior  to  thoaeof  eirole  B,  whiob 
is  the  one  always  nsed  in  astronomical  observation.  Th«  diatter  of 
the  Washington  circle  ia,  however,  about  six^tentha  greater  thaa  that 
of  the  Btrasaburg  instrument,  thua  slMwing  that  in  linear  ■aasnie  the 
general  aeouraoy  la  the  same  in  the  Straaaburg  inatrnmeut  and  the  Wash- 
ington eirole  B. 

I  muet,  in  juatioe  to  the  Measra.  Bepsold,  call  attention  to  the  fiMt  that 
thia  oompariaon  refers  only  to  the  particular  seta  of  division  whieh  are 
distant  6  degrees,  and  doea  not  refer  to  the  general  exoeUoioe  of  the 
dividing.  Both  the  Waahington  cirdes  exhibit  a  moat  anfortanato  pe- 
riodic error  within  eaeh  apaoe  of  5  degreea,  and  another  within  each 
apaoe  of  30  minntee  or  perhapa  one  degree.  In  oinde  B  the  asaxioiun 
amount  of  thia  periodie  error  is  0''.37,  and  in  circle  A,  VMi  I  am  not 
aware  that  any  such  error  exists  in  the  Beps(M  ck«le.  It  ia  proper  to 
remark  that  the  methoda  of  dividing  the  two  inatmmenta  were  eatirely 
different.  In  the  Waahington  instrument  the  original  diviaiona  were 
made  to  every  6  degrees,  and  the  intermediate  onea  am  all  oopiea  of  the 
game  small  dividing  arc  nsed  by  Messrs.  Pistor  and  Martina  fbr  finish- 


UBim 


IN8TRUMKNTH. 


IMPROVEMENTS    IN    ASTRONOMICAL    INSTRLTMGNTH. 


19 


istrnctorB  of  th«  instrn* 
I  might  reach  a  precise 
8trawbnrg  to  collect  the 
r  for  a  rough  coiuparisoii 

lat  these  errors  bad  Iteeu 
liod  of  making  a  numeri  - 
seems  to  be  to  compare 
errors  of  the  two  a4ioin- 
e  table  of  errors  through 
mean  error  as  thus  dc- 
It  will  be  interesting  to 

for  1865  is  found  a  table 
;ton  instrument.    Treat- 

aximnm,  (H'.79. 
ftximnm,  0".31. 
lum,  0".63. 

errors  of  the^StraMborg 
)f  circle  A  of  the  WMh- 
B  thoM  of  drole  B,  which 
ration.  The  dianater  of 
tenths  greater  thaa  that 
lat  in  linear  ■wsnwi  the 
Mtmmeut  and  the  Waah- 

attention  to  the  CMt  that 
ita  of  diviakm  which  are 
(eneral  ezoellMioeof  the 
it  a  moat  unfortanate  pe- 
and  another  within  each 
In  oirde  B  the  Baxinan 
iroleA,0"4e.  I  am  not 
Id  okde.  It  is  proper  to 
[natraments  were  entirely 
he  original  divisions  were 
I  ones  are  all  copies  of  the 
»r  and  Martins  for  finish- 


ranx^ 


iiig  the  divifliouR.  On  the  other  hand,  the  divisions  of  all  the  Repsold 
iiiHtrunieut«  are  copies  of  a  certain  large  divided  circle  made  by  the 
foniiders  of  tbr  tirm  more  than  half  a  oeutnry  ago.  The  copy  is,  how- 
ever, modified  to  thin  extent :  instead  of  each  division  being  copied,  the 
c'upy  is  a  mean  <«f  oach  ftve  (<<>niiecutive  divisions  ou  the  original  circle. 
iSnpposiug  the  copies  U*  be  absolutely  perfect  and  the  original  circle  to 
have  remained  unchanged,  all  the  Repsold  instruments  should  exhibit  the 
«iaiiie  errors  of  division,  but  I  am  uot  aware  whether  the  uumliering  on 
thedifferentcircles  is  made  to  correspond  to  the  samedivisiousof  the  orig- 
iaal  circles,  a  pipceeding  which  would  be  necessary  to  test  the  uniformity. 
Ill  this  connection  I  may  remaric  that  during  my  visit  to  the  Bepsold 
establishment  they  allowed  me  to  examine  the  divisions  on  the  original 
circle  with  the  aid  of  a  powerful  micrometer  microscope  attached  to  it, 
and  used  in  makingacopy.  So  fiir  as  I  conldjudge  from  acursory exami- 
nation there  were  no  sensible  accidental  errors  of  division  in  the  minute 
a]>ace  over  which  my  observations  extended.  If  so,  the  errors  of  division 
in  the  oopy  must  be  mainly  accidental,  and  perhaps  due  to  elasticity  in 
the  mounting  of  the  cuttiiig  tool  combined  with  irreguhkrity  in  the  resist- 
ance it  meets  with  in  cutting  the  metal.  Should  the  errors  of  division  be 
wholly  doe  to  this  cause,  we  could  expect  no  correspondence  betwee^ 
those  on  different  instruments. 

Themioraaoopes  are  about  two  feet  long,  and  their  absolute  power  is, 
I  think,  somewhat  less  than  in  the  Washington  instrument  It  did  uot 
appear  to  me,  however,  firom  examining  the  divisions,  that  they,  would 
bear  any  higher  power  with  adyantage.  The  edges  appear  deficient  in 
straightness  and  sharpness,  and  this  appearance  is  exaggerated  by  the 
numerous  discolorations  upon  the  silvered  surface.  The  probable  error 
of  a  single  setting  of  a  microscope  appeared  to  be  about  double  of  that  in 
the  Washington  drole,  or  0".2  to  0".3  against  (KMO  to  O^Mfi.  From  these 
facts  I  am  led  to  the  oonolnsion  that  an  improved  system  in  the  con- 
struction of  eiroles  is  a  desideratum. 

It  is  true  that  the  necessary  probable  ervm  of  astronomical  observa- 
tions arising  tnm  unavoidable  disturbing  causes  is  such  that  no  great 
additional  aoonraoy  in  single  observations  would  be  obtainable  by  a 
more  aeenrate  reading  of  the  circle.  The  ottfect  of  increased  accuracy 
of  reading  is  to  ftMilitate  the  determination  of  errors  of  division.  The 
latter  must  be  determined  with  a  precidon  corresponding,  not  merely  to 
that  of  a  single  obeervati<»,  but  to  the  mean  of  a  great  number  of  ob- 
servations.   To  do  this  without  an  enormous  expenditure  of  labor,  the 


jwrt^wijmitttwiw^i 


fv'; 

•      ^^ 

■ '. ; :  ^  ■ ,  ■_  "■■ 

V.  - 

-^■^,;::^ 

, ,  '■-<'■ 

"->  ■<  \ ' 

'^•T  }■•■'. 

■  ■_v,^',- 

- -^;  •--■-- 

.      -.L--V                           .■■/ 

■■*..' 

''    .    J 

fc        '^       '"■ 

1  ,.■:-     A  ■ ) 

(#■■'■ 

.■  ■?V'.'-,i 

20 


IMPROVEMENTS   IN  A8TRC50MICAL   INSTRUMENTS. 


microscopes  mast  read  with  sucb  precision  that  a  single  (l^terdlination 
of  each  division  will  snffice.  The  greatest  improvement  in  this  direiBtion 
wonld  be  made  by  the  introduction  of  glass  circles  which  have  lately 
been  proposed  by  several  American  physicists.  The  practicability  of 
this  innovation  can,  however,  only  be  determined  by  experiment. 
Withont  pretending  to  decide,  at  present,  whether  glass  or  metal  will 
prove  to  be  the  best  material,  I  do  feel  that  astronomers  onght  not  to 
rest  satisfied  with  a  degree  of  aecnracy  so  far  behind  that  reached  by 
the  working  physicists,  who  cat  one  or  two  thousand  divisions  to  the 
millimeter,  and  space  them  so  evenly  that  their  inequalities  defy  direct 
n^easorement. 

Ih  tb6  Strassburg'  circle  an  innovation  has  been  made,  designed  to 
render  nnneceesatty  the  determination  of  more  titan  a  limited  number  of 
divisibn  errors.  Oiie  of  the  circles  is  ditid«d  only  to  every  degtvp,  and 
fbut  of  these  degrees,  distftnt  90  degrees  from  each  other,  are  diilded  to 
every  two  minutes.  Thus  there  are  in  all  480  divisions  on  tAie  eircle^ 
and  the  errors  of  these  can  b6  determined  with  great  precision  without 
an  inordina(t6  expenditure  of  labor.  With  the  eirole  thus  divMed  an 
arc  of  any  required  lengtii  can  be  measured,  one  of  whose  tenninf  shall 
lie  in  the  degree  which  is  finely  divided,  and  the  other  on  ob*  of  the 
entire  degrees.  To  do  this  it  is  necessary  to  adjust  the  eIrMe  ou'th^ 
axis  with  each  obeamition  in  such  way  thait  ^at  observation  shall  be 
made  upon  the  finely  divided  part,  while  the  nadir  or  hforiMNotal  point 
shall  IMl  upon  an  entire  degree.  The  latter  point  must  then  twssparaiely 
dMArminied  for  each  astronotfdcal  observation.  I  cannot  tUnk  bat  tbsfe 
the  labdr  of  doing  this  exceeds  the  advantage  gaiLed  by  it 

MXBOUBT  BABin  FOB  KADIB  POOfT. 

I  fouWdiit  Stbas^bbi'lri  I«!j^dMn  atid  ofthibr  oontin«iMaf  <i«Mi^l^M, 
v^i^kH  etf  ibeM^  biHti  tHiich  ilroittt  so  itMfftMIy  tUie  I HA  Mir^ifsed 
at  its  being  almost  unknown  outsid*  of  G^MMtay.  At  tl¥^  tfMbk^- 
t«ry,  ill  hi  ^iHit  i,  iir«ai  dly,  obiMrVattibns  <ff  Wi  mmt  ^Wt  sMd  of 
sRin  Viy  ri^n^Bnra'  i^  i^fikHMrM'  ditttnlt  b^  tStts  tsrtJuiovii  ^MMtMM  by 
#ind,  «(i«  ]()ttiiililif  bf  dk)Mig«^  ikA  thJii  itto^enient  of  iMfi  o^  miam 
ill  tfte  misimimA,  TaHoUti  li^genteM  awft>  cbhii)lii!«ltiJM  ot$iitiH*h^^ 
flliig  in  im  fbt  avbiflih^  thM  dllllotfty,  hone  of  which  ate  eiitiM^  iAti*- 
ftaictbfy.  Th6  ntb  tif  a  cb^p^r  btein  to  hbld  thib  mercery  #iAiiii^»^«ft 
In  Germany  elurly  in  the  ptM^t  cibttti^,  bat  the  oondiftons  li«ici<Mury 
to  rendet  such  a  basin  sticoeiisftil  seem  never  to  have  become  irell  tiii- 


'■WKJa-^  ^'WffS'V 


NSTRUMENT8. 

a  single  tlpteriiiination 
<remeut  in  this  direiotion 
rcles  which  have  lately 
The  practioability  of 
mined  by  experiment, 
ther  glass  or  metal  will 
tronomera  onght  not  to 
>ehind  that  reached  by 
onsand  divisions  to  the 

inequalities  defy  direct 

/■♦v. 

been  madej  designed  to 
uan  a  limited  nnmber  of 
ily  to  every  degmr,  and 
leh  other,  ore  divided  to 
I  divisions  on  fibe  circle, 
great  predsion  without 
B  drole  thns  divided  an 
e  of  whose  terminf  shall 
;he  other  on  ob6  of  the 
accost  the  dMLe  oittM 
lat  obeervatton  shall  be 
mdiror  iMnriMHrtal:  point 
t  must  then  be  ■epamlely 
I  camot  think  bat  that 
i;aii.<!d  by  it. 

POINT. 

ntinefitat  oWWVMtanes^ 
(itiy  tnat  I  Mn  Mtrj^rf sea 
mjt  At  every  ODserva- 
miUMW  tMW  Md  of 
le  cremoTB  pnrawa  oy 
ikelit  of  iMM'Dr  iMniMli 
iohi))lii5titiiid '  otSulnf^uSblMi^ 
tHiIeh  ate  eUtlM^  i&ti«- 
« inera^ty  lhii^1ii«^«d 
the  oondit!biis  liCio^Mury 
:o  have  beeome  irciH  xti- 


IMPROVEMEMTS   IN   A8TR0N011ICAL   IN8TBUMENT8. 


21 


derstood.  In  its  most  improved  form  the  basin  has  no  sides,  bnt  is 
simply  a  circular  plaitie  of  metal,  of  ^ioh  the  upper  surface,  instead  of 
being  perfectly  flat<,  is  slightly  concave,  the  figure  of  the  concavity  being 
spherical.  The  depth  at  the  center  is  but  a  fraction  of  a  millimeter. 
The  surface  is  copper,  or  the  whole  basin  may  be  made  of  copper,  be- 
cause in  contact  with  this  metal  the  mercory  does  not  roll  into  globules, 
atieast  when  amalgamated  with  it.  In  use  the  topof  the  basin  is  approxi- 
mately leveled  with  a  spirit-level.  A  little  mercury  is  then  poured  upon 
it,  and  tbe  surface  of  the  mercury  is  gradually  brushed  off,  partly  to 
remove  dust  and  imparities  and  partly  in  order  to  get  rid  of  the  antplus 
mercury.  There  is  th«i  left  a  layer  of  mercury  in  the  center  of  the 
basin  so  thin  that  waves  cannot  oontinue  on  its  snr&oe.  At  Leyden, 
Professor  Van  de  Sande  BidchuyieD  gave  me  ao  opportaaity  to  make 
v«ry  carefkl  experiments  on  the  workmg  of  tkia  system.  The  spkerioity 
of  the  basin  gnarte  against  what  haa  been  considered  one  of  tke  dangers 
in  using  an  anangement  of  this  (kind,  namely,  a  possible  aiiNite  indi- 
natkm  of  the  liquid  sorfiMe  arising  from  the  eohedoo  between  the  mer> 
only  and  the  copper  of  the  eontaining  veaseL  I  found  that  in  making 
flD'Obeewation  of  the  nadir  point  the  heavy  walking  of  men  avannd  the 
reogoi  produced  no  dietorbance  whatever,  and  evMi  stamping  on  the 
floor  ia  the  neighborhood  of  the  inatrament  only  caused  a  monentary 
diwippearenee  «t  ti»  refleotM  image  of  the  wirea 

la  the  aftplioatioB  of  titia  form  of  biMun  it  is  desimble  that  the  plate 
ahoold  be  considerably  larger  tlian  the  olgective  in  order  that  the  layer 
of  mercury  may  be  equal  to  the  latter  and  yet  have  plenty  of  margin 
aMmnd  it  oh  the  plate. 

MHrraoM  OP  SBTSBMiMnro  plbxub%  bio. 

In  the  use  <tf  a  vertical  oinde  of  any  sort,  when  the  highest  precision 
is  aimed  at,  the  determination  of  the  el&ots  produced  by  the  bending 
«f  the  instmment  in  the  difGerent  positions  has  always  been  one  of  the 
most  difiioalt  problems. 

The  determination  of  the  horiaontal  flBzare  is  commonly  oonsidMed 
to  oflhr  no  dilBoalt^.  The  &miUar  method  of  setting  the  opposing  col- 
limators in  soeh  a  position  tliat  light  can  pass  from  one  to  the  other 
through  an  opening  in  the  central  cnbe  of  the  meridian  oiMe,  and  then 
obaewing  the  horiaontal  wires  in  each  of  them  with  the  drde,  is  of 
oniversal  i^nnlication.  But  even  in  the  dmplest  case  inegnlarities  and 
disoordanoes  are  frequently  found.    In  the  Washington  drde  I  traced 


22 


UIPBOVEMENTS   IN   ASTRONOMICAL   IN8TBUMENTS. 


,  •>,  )■'■'. 


1 

.      f. 

;    ;  ' 

■^ 

-•^.i 

,j 

.'•  ■■■  ■  1 

■ "  '■  '^^-i 

^>'".. 

'?.^      * 

•    1 

»i 

'%'  \ 

'■  ,  i,"" 

.^'} 

T'l 

y.  /i' 

^ 

'■-  n  > 

:■:'"-■ 

1  ■  '■ 

fe 

V       '- 

*:'., 

-/■.'-', 

-w»r- 

:,^-^, 

1 ».-,-  "■■ 

1'^ 

:% 

^';;i 

these  disoordanous  to  the  effect  of  refraotioo  caused  by  the  diftiere  nt 
temiteratares  of  different  strata  of  the  air  in  the  observing  room  and  in 
the  instrument.  It  is  therefore  absolutely  necessary  to  success  that  the 
determination  should  be  made  when  there  is  a  perfect  uniformity  of 
temperature  inside,  around,  and  above  the  instrument.  This  was  found 
practicable  only  in  periods  of  long-continued  rain,  which  cooled  off  the 
roof  of  the  building  and  thus  prevented  an  accumulation  of  warm  air 
in  the  upper  part  of  the  room. 

This  method  has  generally  been  employed  only  when  the  instrument 
was  horizontal,  and  its  application  in  other  positions  is  so  troublesome 
that  it  has  seldom  been  undertaken.  At  Paris,  however,  I  found  in  use 
on  the  new.Bischoffisheim  circle  a  vertical  collimator  which,  although 
apparently  not  intended  for  that  purpose,  could  readily  be  used  to  deter- 
mine the  flexure  in  a  vertical  position.  The  collimator  itself  is  supported 
in  a  horizontal  position  upon  a  standard  on  the  east  pier,  around  which 
it  turns  upon  a  vertical  axis.  When  placed  in  position  for  use  its  ob- 
jective is  over  the  center  of  the  telescope.  To  receive  the  rays  from  the 
latter  a  reflecting  prism  is  placed  in  front  of  its  objective.  Thoa  the 
result  is  optically  the  same  as  if  the  collimator  looked  vertically  down 
into  the  telescope  of  the  main  instrument.  There  would  be  no  diffloolty 
in  setting  the  collimator  either  upon  its  reflected  image  in  the  baaiM 
of  quicksilver  below,  or  on  another  vertical  telescope  below  the  floor. 
The  apparatus  would  then  be  available  for  the  determination  of  flexure. 

Mlk  lokwy'8  method  of  mbasubino  flbxubb. 

An  ingenious  plan  has  recently  been  proposed  by  Mr.  Loewy,  vioe-, 
director  of  the  Paris  Observatory,  for  determining  the  flexure  of  the 
telescope  in  all  positions.    It  has  been  sa  fully  described  in  the  OompiM 
Rwdfu  and  other  publications  that  I  need  only  here  give  its  general 
principle. 

A  small  glass  instrument  which  combines  the  function  of  a  lens  and 
a  reflector  is  placed  in  the  central  cube  of  the  telescope.  The  flexures 
of  the  two  ends  of  the  telescope,  relatively  to  the  reflecting  surface  of 
this  glass,  are  separately  and  independently  determined  in  all  posttionif  of 
the  instrument  It  is  assumed  that  the  glass,  b^ing  in  the  neutral  axte 
of  the  telescope,  the  astronomical  effect  of  the  flexure  will  be  given  b;^ 
the  diffierence  in  the  flexures  of  the  two  ends  relatively  to  the  glass. 
Ingenious  and  well  considered  as  this  method  is,  I  cannot  consider  it' 
reliable  for  determining  the  effects  of  flexure,  because  it  leaves  out  of 


mmm 


I8TBUMEMTS. 

kosed  by  the  difterent 
observing  room  and  in 
ary  to  snccesa  that  the 
perfect  uniformity  of 
uent.  This  was  found 
i,  which  cooled  off  the 
imulation  of  warm  air 

ir  when  the  instrument 
dous  is  so  troublesome 
lowever,  I  found  in  use 
nator  which,  altliough 
eadily  be  used  to  deter- 
ator  itself  is  supported 
last  pier,  around  which 
position  for  use  its  ob- 
loeive  the  rays  from  the 
)  objective.  Thus  th« 
Looked  vertically  down 
I  would  be  no  diffloulj^ 
d  image  in  the  basin 
»8cope  below  the  floor, 
itermination  of  flexuce. 

a  FLEXUSB. 

3d  by  Mr.  Loewy,  vice-, 

ing  the  flexure  of  the 

9Soribed  in  the  Oompte* 

here  give  its  general 

(hnction  of  a  lens  and 
slesQope.  The  flexures 
he  refleeting  surfiioe  of 
mined  in  all  posttioncf  of 
iing  in  the  neutral  axiik 
lexnre  will  be  giv«n  bf 
relatively  to  the  glass, 
is,  I  oannot  consider  it 
Ijecanse  it  leaves  out  of 


IMPBOVEMENTS   IN   A8TB0N0MICAL   INSTRUMENTS. 


23 


mmmm*' 


account  the  bending  of  the  parts  of  the  instrument  between  the  central 
cube  and  the  divisions  on  the  circle.  In  the  Washington  circle  there  is 
a  well-marked  flexnre  of  the  circle  itself,  which  may  be  expressed  by 
saying  that  if  the  central  cube  revolves  uniformly  the  circumference 
of  the  circle  does  not  revolve  uniformly  but  is  affected  with  a  periodic 
inequality.  Hence,  to  make  the  determination  tree  firom  all  sources  of 
error,  the  flexure  must  be  determined  by  a  direct  comparison  of  the  op- 
tical axis  of  the  telescope  with  the  reading  of  the  circle  divisions  under 
the  microecopeti.  This  can  be  done  only  by  pairs  of  opposing  collima- 
tors on  the  usual  or  Besselian  plan,  or  on  some  other  plan  by  which  rays 
can  be  sent  in  the  same  straight  line  in  two  opposite  directions. 

OOLLDIATOBS  AND  MSBISIAN  MASKS. 

l*be  old-fashioned  system  of  placing  meridian  marks  at  such  a  dis- 
tance that  they  could  be  observed  tiirongh  the  telescope  of  the  transit 
circle  without  changing  the  astronomical  focus  may  be  regarded  as  now 
entirely  abandoned,  owing  to  the  bad  effect  produced  on  the  images  by 
the  passage  of  the  light  through  several  miles  of  air  near  the  ground. 
The  present  system  is  to  place  the  meridian  miurk  at  a  distance  of  100 
or  200  yards  and  to  render  the  rays  emulating  flrom  it  parallel  by  a 
lena  of  long  focus.  The  plan  of  putting  this  lens  as  a  cap  over  the  ob- 
jective has  be«i  abandoned,  owing  to  its  displacement  of  the  optical 
center  of  the  combined  system  of  the  lens  and  the  ol|jeo^gla88.  It 
is  therefore  usaally  fixed  on  top  of  a  pier.  Bnt  at  Straasbnrg  a  differ- 
ent system  is  adopted.  No  lens  of  long  Ibons  is  osed  at  all,  bat  fhe  tel- 
escope  is  pointed  directiy  upon  the  waridian  mark  and  the  rays  are 
brought  to  a  focus  in  the  plane  of  the  spider  Unes  by  means  of  a  lens 
of  short  focus  which  can  be  slid  into  tiie  «ye-pieoe  of  the  tdesoope.  On 
this  plan  the  position  of  the  image  will  depend  upon  that  of  the  small 
lens— a  dependence  which  I  think  ought  to  be  avoided.  The  best  sys- 
tem seems  to  me  to  be  that  of  the  fixed  objective  of  long  focus. 

At  Strasabnrg  the  fixity  of  the  meridian  mark  is  aMoied  by  support- 
ing it  on  a  very  firm  stone  foundation  and  protecting  it  by  a  frame  build- 
ing from  the  rays  of  the  ran.  The  neoeasitiy  of  these  inecaationB  is  too 
obvious  to  require  any  comment  upon  them. 

At  Paris  a  very  ingenious  system  of  refiecting  collimators  is  applied 
to  the  great  transit  circle.  As,  however,  this  system  was  probably 
adopted  only  because  there  was  no  room  for  collimators  of  the  usual 
construction,  I  did  not  deem  it  necessary  to  prepare  a  description  of 


u 


IMPROVEMEINTS  IN  A80SONOMICAL  IN8TSUM£MTS. 


them.  A  system  of  fiducial  lines  which  may  be  adopted  with  advantage 
in  ordinary  collimators  is,  however,  worthy  of  note.  Instead  of  setting 
one  spider  line  upon  the  image  of  another,  the  spider  line  in  one  colli- 
uiator  is  replaced  by  a  fine  transparent  line  through  a  narrow  band  of 
some  oiiaque  substance  on  a  plate  of  glass.  Thus  when  the  observer 
looks  at  the  image  of  this  band  in  the  other  collimator  he  sees  in  the 
center  of  the  field  a  fine  horizontal  or  vertical  bright  line  ou  which  he 
can  set  the  dark  line  of  bis  collimator  with  great  precision.  This  plan 
does  not,  however,  so  far  as  I  can  see,  readily  permit  th«  setting  to  be 
made  by  means  of  the  dark  band  collimator.  Whether  this  limitation 
is  a  serious  defect  is  a  question  on  which  opinions  may  differ. 

OB8BRVAT0BT  BUILDINGS. 

In  the  coarse  of  my  journey  I  had  the  opportunity  of  visiting  two 
new  observatories  of  the  first  class  erected  within  the  past  law  years: 
one,  the  Imperial  Observatory  at  Yienna,  the  other  the  Astro«Physi«al 
Observatory  at  Potsdam.  It  is  generally  necessary  to  design  aa  •b- 
servatory  with  especial  reference  to  the  eharactor  of  the  observatians 
to  be  made  and  the  objects  to  be  parsued.  To  this  may  be  add«d  the 
frequent  neoeasity  Ibr  gratifying  some  public  taate  witb  leapeet  ti>>arohi- 
tectnre.  For  these  reasons  one  observatory  cannot  weU  serve  as  a  oodel 
for  another;  bat  there  are  certain  special  features  which  would  work 
equally  well  under  neariy  all  conditions,  and  whioh  ace  therefore  worthy 
of  oonsiderAtion  in  building  any  observatovy. 

In?the  Yienna  Obaervatoij  tiie  arehitactoral  Meatent  predeminafles. 
I  did  not  observe  any  new  featuie  of  eapcoial  impovtaiMe  to  the  deaigii- 
era  «f  ftitare  observatories  exioept  those  already  noted. 

■The  Potsdam  Obserroftofy,  as  its  name  implies,  was  desifaedfwith 
especial  vefennce  to  physical  tribserrakions  upon  the  kewrenly  bodies. 
This  braaeh  of  astrmomy  in  its  pnsent  developsMiit  te  |M  new>tiiat 
every  estebUshment  ibr  proeecating  it  has  ts  be  ^aoned  with  refersooe 
to  the  speotelw«riE  to  be  doae.  Hence,  nstfrtlhstonding  thufeldM^ob- 
serratoty  in  qoestion  is,  in  itsowtit  asddssign»  ons  sf  ths  most  pegftst 
yet  bnilt, those  featnies  of  it  wUoh  it  woald 'be  advisabls  toineofpo- 
rate  in  another  establishment,  baiIt,periM^  for  another  purpose,  would 
generally  ooour  to  the  designers  of  sasbaa  establishment  The  following 
are,  however,  well  worthy  of  ooasideration  in  all  plans  of  new  observa- 
tories. 

The  sffiBct  of  the  sun's  rays  npoa  the  metal  roof  of  the  baiUing  is  to  hsat 


I 


I 


mrmmm 


mmm 


advantage 
1  of  setting 
k  onecoUi- 
»w  band  of 
6  observer 
aees  iu  the 
I  which  he 
This  plan 
ttingtobe 
limitation 


Biting  two 
bw  years : 
NPhysieal 
gn  aaob- 
mvatioiis 
idd«dthe 
tto>ardii- 
tsamodel 
mid  work 
re  worthy 

Nninales. 
ttd«aigB< 

raed<^witti 
lyboditt. 
new>ttet 
refirMioe 
*tk»  ob- 
it poefiwt 
ineorpo- 
le^wodid 
following 
obnerva- 

istohoat 


IMPBOVEMEMTS  IN  ASTRONOMICAL  INSTRUMENTS. 


25 


I 


I 


the  air  in  immediate  contact  with  the  roof  and  thns  to  injure  the  definition 
of  the  heavenly  body  seen  firom  within  the  observing  rooms  in  the  day- 
time. To  avoid  this  difficnlty  the  roof  is  covered  with  soil  and  sodded 
with  green  turf  and  thus  kept  as  cool  as  the  surrounding  air,  how  hot 
soever  the  sun's  rays  may  be.  It  is  of  course  necessary  to  keep  the 
turf  watered.  Possibly  any  other  absorbent  substance  might  answer 
the  purpose  of  the  turf,  but  the  latter  has  at  least  the  advantage  of 
cheapness. 

It  is  a  common  feature  of  all,  or  nearly  all  the  continental  observa- 
tories, that  quarters  are  provided  for  the  astronomers,  generally  in 
the  building  itself.  This  offers  the  great  advantage  that  the  astron- 
omers are  nearly  always  near  their  instruments,  and  may  be  regarded 
as  absolutely  essential  to  the  efficiency  of  any  large  observatory,  espe- 
cially if  it  is  not  in  the  midst  of  a  city.  At  Potsdam  the  houses  of  some 
of  the  astronomers  are  separated  from  the  building,  but  the  general  rule 
is  that  the  building  accommodates  the  director  and  such  of  the  assist- 
ants as  are  engaged  in  actual  observations,  together  with  their  fomilies. 
This  collection  of  several  fiunilies  in  the  same  building  is  more  accord- 
ant with  European  habits,  than  with  our  own,  and  the  question  of  its 
introduction  among  us  can  be  settled  only  by  careful  consideration.  I 
can  only  say  that  I  noticed  no  serious  inconvenience  arising  firom  it. 

GLOOXS. 

In  most  astronomical  work  of  the  first  dass^  especially  in  meridian 
observations,  the  perfection  of  the  dock  is  as  necessary  as  that  of  any 
other  installment  But  it  seems  to  be  an  observed  fact  that  no  certain 
way  has  yet  been  found  of  securing  an  approach  to  perfection  in  the 
rate  of  the  dock.  All  we  can  say  is,  that  dooks  of  marvelous  excellence 
are  now  md  then  made,  sometimes  by  one  maker  and  sometimes  by  an- 
other,  and  that  of  these  docks  some  are  permanentiy  good  while  others, 
in  the  ooune  of  time,  deteriorate.  I  found  a  few  examples  of  docks  pre- 
serving their  rate  with  remarkable  uniformity  through  oonsiderable  pe- 
riods. One  of  these  is  the  Normal  dock  of  the  Berlin  Observatory,  made 
by  Tiede.  It  is  indosed  in  an  air-tight  case  in  wder  to  prevent  changes 
of  rate  arising  from  variations  in  the  barometric  pressure.  The  tem- 
perature compensation  is  unfortunately  imperfect,  so  that  the  rate  is  sub- 
ject to  an  annual  change.  This  fact  has  prevented  the  exact  discussion 
to  which  I  desired  to  subject  it  It  would  seem,  however,  ifcom  a  cur- 
sory examination,  the  materials  for  which  were  courteously  afforded  me 
S.  Ex.  96 3 


WS  IMPBOVSMENTS  IN  ASTBONOMIOAL  INSTRUMBNTS. 

by  Professor  Fdr»ter,  that  the  aanaal  change  from  temperaturo  does 
not  exceed  10  or  15  seoonds  per  year,  and  that  when  this  is  allowed  for 
the  differences  between  the  actual  and  the  computed  errors  will  be  a  very 
few  seconds  per  year.  In  recent  times  the  docks  Aimished  by  Bowhtt, 
of  Amsterdam,  have  secured  a  reputation  for  uniform  excellence  which 
has  never  been  surpassed ;  that  is,  instead  of  being  able  to  occasionally 
turn  out  a  dodk  of  remarkable  excellence,  all  the  clocks  of  this  artist,  so 
far  as  they  have  been  discussed,  are  of  the  first  class. 

The  following  exhibit  of  the  observed  and  computed  errors  of  one  of 
his  clocks  through  a  period  of  nearly  two  yean  has  been  selected,  not 
from- a  belief  that  this  particular  clock  was  better  than  others,  but  be- 
cause the  data  toe  the  examinati<m  were  at  hand : 

ComparUon  of  the  obiarwd  and  eomputed  eometioiu  of  Cloek  HoukU  9;,  at  fk0  oh$ervatory 
^£<y4m,  186^2)MWM>Mrl,  toiaer,  OoM«-95. 

[FopMUUi  roB  courirtED  dailt  mu*-  +fl^-'IS»  -  <I>.MM9  <T — 14P)  +  0*.01M  (B — 7Mw). 
T  =  tempentnre,  cent. :       . 
B  «  kaigbt  of  bMoaiAter.] 


Correction. 

V 

Observed. 

Computed. 

Difference. 

1865. 

a. 

«. 

$i 

Deo.     1 

90.9 

90.9 

0.0 

99 

56.3 

54.8 

+  1.5 

1866. 

Jan.    96 

89.1 

84.9 

+  4.9 

F«b.   93 

119.8 

119.1 

+  7.7 
+13.1 

Mu.   30 

961.1 

148.0 

Apr.    97 
Mk7   95 

188.9 

179.5 

+1&4 

915.3 

195.5 

+19.8 
+00.5 

Jane  99 

938.5 

916.0 

July  37 

949.7 

931.7 

+18.0 
+19.7 

Aog.  SI 

965.0 

9nl.3 

^U 

377.9 

993.4 

+  7.9 

997.4 

+  4.0 

Nov.  30 

397.8 

390.5 

+  1.3 

Deo.   98 

356.9 

355.9 

+  1.0 

1807. 

. 

Jan.    95 

385.9 

386.5 

—  1.3 

Fab.   99 

414.3 

418.1 

—  1.8 

Mar.  99 

459.3 

^1 

454.5 

—  9.3 

Apr.   96 

479. 1 

—  3.0 

50&.8 

S06.3 

+  0.5 

Jane  98 

694.7 

589.9 

+  1.8 
+  1.4 

Jidy  9G 

540^8 

539.4 

Ang,  30 

559.9 

559.5 

+  0.4 

8api.97 

875.4 

577.9 

—  1.8 

Oct.    95 

594.6 

600.6 

-6.0 

In  this  connection  I  may  be  allowed  to  call  at^onihn  to  the  unsatis- 
factory character  of  the  data  usually  presented  tor  estimating  the  ex- 
cellence of  clocks.    In  my  judgment  the  estimate  of  the  clock  should 


iii 


I 


:.l 


IMPROVEMENTS  IN  A8TB0N0MICA.L  INSTRUMENTS. 


27 


ture^doee 
lowed  for 
be  a  very 
r  Bowhii, 
loe  which 
Mionally 
arti«t,fM> 

of  one  of 
loted,  not 
I,  bat  be- 

»b$enatory 


P 


1 


e  onsatis* 
kg  the  ex- 
ck  shoald 


be  foonded  upon  its  errors,  determined  from  time  to  time  through  a  pe- 
riod of  not  less  than  a  year.  These  errors  should  be  exhibited  in  coD' 
nection  with  the  mean  temperature  of  the  clock-room,  and  if  the  dock 
is  not  in  an  air-tight  case  the  height  of  the  barometer  should  also  be 
given.  A  calculated  error  should  then  be  carried  through  the  whole 
period,  in  which  the  corrections  for  temperature  and  height  of  the  barom- 
eter should  be  introduced.  A  clock  which  stands  this  test  well  may  be 
presumed  beyond  doubt  to  keep  its  rate  during  short  intervals,  which 
is  generally  the  important  point. 

It  is  very  common  to  present  as  sufficient  data  forjudging  of  a  clock 
an  exhibit  of  its  daily  rates  firom  time  to  time.  If  these  rates  were  really 
determined  with  the  last  degree  of  accuracy  they  might  be  sufficient 
for  the  purpose.  But  as  found  in  practice  they  will  be  the  result,  not 
merely  of  the  actual  rates  of  the  clock,  but  of  various  personal  differ- 
ences among  the  observers  and  changes  in  the  pointing  of  the  instru- 
ment as  well  as  the  accidental  errors  of  observation.  From  these  causes , 
although  the  clock  were  perfect,  we  might  expect  an  apparent  difference 
of  several  hundredths  of  a  second  between  its  apparent  rate  on  succes- 
sive days. 

The  barometric  change  in  the  rates  of  all  clocks  of  the  usual  construc- 
tion is  so  important  a  drawback  that  it  shoald  no  longer  be  tolerated 
in  work  of  the  first  class.  Two  methods  have  been  proposed :  the  one, 
that  already  mentioned,  of  inclosing  the  dock  in  an  air-tight  case;  the 
other,  to  supply  it  with  a  baromelric  OQmpensation.  The  latter  method 
is  undoubtedly  the  easiest,  but  where  the  necessary  perfection  of  ar- 
rangements can  be  secured  the  former  must  be  considered  greatly 
preferable.  The  grounds  of  preference  are  that  the  air  can  be  exhausted 
from  the  case  to  any  extent,  thus  diminishing  its  resistance  to  the  mo- 
tion of  the  pendulum  and  permitting  a  diminution  in  the  driving  power. 
Again,  if,  instead  of  air,  the  case  be  filled  with  some  gas  which  does  not 
act  on  the  oU,  the  slow  oxidation  of  the  latter  may  be  prevented^  It 
may  therefore  be  expected  that  under  this  system  a  clock  oould  be  al- 
lowed to  remain  undisturbed  for  a  longer  period  than  under  any  other. 
Yery  respectfliUy,  your  obedient  servant, 

SIMON  NBWOOMB, 

ProfMsor,  United  8tate$  Kwvy. 
Hon.  W.  E.  Ghandlbb, 

Seeretary  of  the  Nary. 


mm 


p«* 


